There's a tl;dr at the bottom. I learned a lot of new things as I wrote this (such as Bestial Fury not working with Two-Weapon Fighting or with Crossbow Expert's bonus action attack...), so I edited this a LOT while writing it and may have missed something. The conclusions are accurate though.
Small size Beastmaster
Why Small size? Because you can ride your Animal Companion and take the Mounted Combatant feat. That doesn't sound like much, but it's actually quite significant. Pets have relatively low HP (4HP per Beastmaster level) and a crap AC (until you invest in Barding way down the line), so Mounted Combatant will keep it alive much longer. It also gives you a boost to speed and could give you (if the DM allows it) access to flight right at level 3.
I. Races
Gnome or Halfling.
Gnomes get better racials IMO. Rock Gnomes don't do much for you, but Forest Gnomes fit thematically and give you a lot of goodies. You get +1 DEX, you can talk to small animals, you get the Minor Illusion cantrip for free (this is awesome), and you get Advantage on INT, WIS, and CHR saves vs Magic (this is even more awesome). Deep Gnomes are an okay option too, but only with the feat they get. Since the builds below are somewhat feat-heavy, you can't take the optional magic feat, making Deep Gnomes less optimal.
Halflings get +2 DEX, which is amazing. But point buy only guys up to 15, so you still can't hit 18 to get +4. Yeah, it's 1 DEX more than Forest Gnomes, but for that you give up a lot. Their Lucky trait isn't as good as the feat, but it's still great because it will prevent Natural 1s (honestly, those kinda make the game fun though...). If you have to go Halfling, I'd go Stout for the CON and poison resistance. Lightfoot doesn't give you anything worthwhile.
Forest Gnome looks to be the most optimal if you're going point buy. Stout Halfling and Forest Gnome are both optimal choices if you can roll your stats and break the +3 mod barrier. I'd still lean towards Forest Gnome though, because the racials are just that much better. If you roll, and your highest number is even (14, 16, or 18), then I'd consider the Stout Halfling better at that point because it'll potentially open up a feat later on.
II. STR vs DEX
Point buy STR: STR 14, DEX 14 (13+1), CON 12, INT 12 (10+2), WIS 14, CHR 10
Point buy DEX: STR 8, DEX 16 (15+1), CON 14, INT 12 (10+2), WIS 14, CHR 10
DEX comes out ahead.
III. Weapons
Somehow, Small PCs are capable of carrying Lances. For STR, Lance and Shield sounds good. Highest damage (d12) and still boost AC, right? This requires the Shield Master feat to be good though, because without shoving a foe back to 10ft range or Prone, you're gonna have Disadvantage a lot... There are some pets that can Prone though too, so we'll have to see. Your Small size means no Heavy weapons, so you're limited to weapons like Longsword (d8 or d10 versatile). Not as optimal as the Lance, but it doesn't require a feat. If you want to take the Dual Wielder feat, two Lances (d12*2) would be hilarious. Unfortunately, without the Shield Master shove combo, you're going to have Disadvantage pretty often with that too. Two Longswords (d8*2) would be more reliable. Apparently Bestial Fury doesn't work with Two-Weapon Fighting though, so you can't do both. I'll do the math later in the Builds section.
My first instinct with DEX is to go Ranged, but again, our Small size means no Heavy weapons like Longbow or Heavy Crossbow. Fighting Style gives you a few good options. Archery style Light Crossbow's damage (d8) is equivalent to a Rapier until Crossbow Expert is picked up at level 4, giving you 2 Hand Crossbows (d6*2). Two Weapon Fighting style Rapiers (d8*2) looks attractive, but again, can't Bestial Fury with them. Dueling style with a Rapier (d8) and Shield if you really want the AC, but damage doesn't look as good as the others. It might work if your Bestial Furies come out ahead of Extra Attack and a bonus action Attack though. Again, I'll do the math below.
IV. Feats
Mounted Combatant looks necessary regardless of your build. Your Small size and Medium mount nerfs the Advantage on attacks vs smaller enemies feature, but the ability to "tank" for your low HP pet is crucial regardless. This will help significantly at higher levels, since your pet's HP scales poorly (40HP at level 10 means a single Dragon's Breath will 1-shot it)... Ranged characters will take less attacks though, so maybe it doesn't have to be rushed for ranged builds.
Shield Master is a must if you're going Lance and Shield. Lance has Disadvantage at 5ft range. Shield Master lets you shove enemies either Prone or into 10ft range as a bonus action before attacking them with the Lance, negating the Disadvantage. Note: Prone won't give your Once you get Bestial Fury, you have to start shoving Prone instead of knocking them back because the Bestial Fury requires you to be in 5ft range. The only good exception to this is Giant Poisonous Snake, because it has a 10ft range. We'll go more into that later though.
Crossbow Expert looks interesting. If you really want a ranged character then this looks really good. You can free-load the same Hand Crossbow twice to attack with both action and bonus action, and still hold a Shield for the extra AC. (Common sense says you would need a free hand, but this feat specifically negates Loading aspect of crossbows so it's RAW!) Unfortunately, RAW doesn't give you the bonus action attack unless you take the Attack action, so you can't Bestial Fury and still get the extra attack. That's a big deal when you see just how much damage some pets can put out by themselves.
Sharpshooter will boost the crap out of your ranged damage. More useful when you can get Advantage. At level 7, Rangers can command their pet to Help for Advantage as a bonus action. I smell synergy.
Dual Wielder gives you 2 Rapiers (d8*2) and still boosts your AC for melee DEX users. For STR builds, this opens up Double Longswords (d8*2). As I said before though, dual Lances (d12*2) sounds amazing, but you will have Disadvantage often without the Shield Master shove combo. Also, Two-Weapon Fighting requires you to take the Attack action, so this doesn't synergize with Bestial Fury, eliminating a LOT of good build options...
Defensive Duelist could be nice for certain builds. Makes you really tanky even without a Shield, but requires a Finesse Weapon. Opens up extra non-Shield build options, because you can still tank for your pet with Mounted Combatant and this feat.
Lucky is good for everyone. Great for Lance builds that need the Advantage as well as ranged builds that want Advantage for Sharpshooter.
Sentinel and Polearm Master synergize well, and you can try a Quarterstaff build around this. Giant Poisonous Snakes even have a 10ft reach, so you can Bestial Fury with them at 10ft range. Unfortunately, to get the damage boost from Polearm master, you have to take the Attack Action rather than Bestial Fury, so this is a mess. Fortunately, Hunter's Mark kicks in on that extra attack and makes it worth it. Kinda. I'll dedicate a build below to this to see what's up.
V: Pet
Ask your DM if you can use the Animal Friendship spell to "upgrade" your Animal Companion to something strong you run into in your travels with a higher CR than 1/4. The answer will probably be no, but it can't hurt to ask.
Note: You get to add your Proficiency bonus to pet attacks, but the damage listed in this section doesn't include that. See the Builds section for complete math.
Melee options:
Giant Badger - 1d6+1 = ~4 damage. Looks weak at first, but two things can change that significantly: 1) if your DM allows Multiattack (RAW), and 2) if you get to add your Proficiency bonus to each attack of that Multiattack (unclear). If the DM is cool with both of these things, then the badger will out-damage other pets and should replace the snake below in Bestial Fury builds (that don't rely on the snake's 10ft range). I'm assuming a lot of DMs won't allow proficiency stacking, so I won't even bother making builds around this pet. Can't hurt to ask your DM though!
Giant Poisonous Snake - 1d4+4+3d6(/2) = ~17(12) damage. Holy hell. This is twice as good as some of the other pet options. The DC 11 CON save is easy to make though. We'll say any roll of 5 or less on a d20 fails the save, so when doing the math, we can say there's a 25% chance to fail the save. When you do the math, (12+12+12+17)/4 = 13.25. For sake of ease, we'll just say the snake averages ~13 damage per attack. Also, with +6 to hit, it's very accurate! This pet is the bread and butter of Bestial Fury builds. Oh, and did I mention it has a 10ft reach? 10ft range synergizes with Lance, Quarterstaff+Sentinel, and Ranged builds.
Panther - 1d6+2 = ~6 damage. - Pounce is nice because Prone. Prone looks particularly good for Lance users because the Advantage on a Prone enemy negates the Disadvantage from being in 5ft range, allowing you to get your pet in range for Bestial Fury. It's kind of a trap though. The Panther damage is crap compared to your Lance attacks, especially with modifiers and Hunter's Mark. Pouncing does potentially increase damage on the first round (1d6+2 + 1d4+2 = ~10 damage), so Bestial Fury Pounce for that initial Prone is actually an effective tactic. I wouldn't Bestial Fury after that though.
Wolf - 2d4+2 = ~7 damage. Pack Tactics gives it Advantage 100% of the time while you ride it, so despite lower damage, it's actually an okay option for Bestial Fury builds. Speed 40, Stealth bonus, free chance to Prone. Not optimal most of the time, but this is a solid backup choice at all levels.
Ranged options:
Pteranodon - 2d4+1 = ~6 damage. Flight. Best option for Ranged users because there won't always be walls to climb when you want to get away.
Giant Wolf Spider - 1d6+1+2d6(/2) = ~12(8) damage. Same 25% poison chance as with the snake. (8+8+8+12)/4 = ~9 damage average. That's pretty darn good! Wall climbing, Stealth bonus, speed 40, poison. Good for ranged builds. Another solid Bestial Fury backup, though personally, I like the Advantage from Pack Tactics more.
Other options:
Giant Frog - 1d6+1 = ~5 damage. Free Grapples. Swallowing is hilarious. Optimal choice for fun?
Giant Crab - 1d6+1 = ~5 damage. Naturally high AC, but Barding and Mounted Combatant make that less useful. You can safely send it in alone for ranged builds to Help you for Advantage when Sharpshooting. The Grapple feature makes this a great option for tank builds too.
tl;dr It's hard to pick the best pet because it's so campaign-dependent and every build is different. Looks like Giant Poisonous Snake would amount in the most damage for melee builds that want to use Bestial Fury. Panther and Wolf are tactical for Lance users because of Prone. Giant Crab is good for tank builds that don't need high damage. Pteranodon if DM allows flight, else Giant Wolf Spider for kiting ranged builds. Giant Crab for ranged builds where you send your pet in alone to Help (don't ride it). Giant Poisonous Snake for ranged builds that still want to use Bestial Fury (don't ride it).
VI. Spells
Cure Wounds/Goodberry for heals. A heal spell is always good as a backup.
Hunter's Mark is great as a first round spell, doesn't synergize well with Bestial Fury (because you're losing Extra Attack), but lasts the entire encounter. Unless you're riding a Pteranodon, I don't recommend casting it with a higher level slot, even though it could potentially last all day. Why? Because it requires Concentration, and you're getting hit or tanking for your pet often enough if you aren't ranged.
Pass Without Trace is amazing for you, your pet, and your teammates. Take it.
Stoneskin, especially at 15 when it also works on your pet, can be good for Bestial Fury tank builds. Losing Hunter's Mark for this isn't a big deal when most of your damage is coming from your pet anyway.
Swift Quiver is good for some builds and bad for others. First of all, you don't get it until level 17. Frankly, you should never build a character around mechanics that don't kick in for the first 16 levels of your game. So if you're starting below level 10, pretend this spell doesn't exist, because chances are you'll never see it in action anyway. Crossbow Expert already gives us a free bonus action attack, but we won't use that bonus action attack because Bestial Fury doesn't synergize with it. So comboing with Bestial Fury is a good use of this spell. It combos well with Sharpshooter too, especially vs low AC foes. Unfortunately, not a lot of enemies will have a low AC at level 17... So yeah, it depends on if your build makes use of Bestial Fury and/or Sharpshooter.
VII. Multiclassing
Barbarian 1/2 - Rage synergizes with Bestial Fury (doesn't require Attack action). Only for STR builds. Reckless Attack would work well for Lance builds.
Lore Bard 3 - Cutting Words on saves like Poison and Grapple. More spell slots.
Lore Bard 6 - Lmfao. Additional Magical Secrets for Enlarge or Haste. Enlarge to make a Giant Giant Frog to Swallow Medium size foes or Haste it to Grapple and Swallow on the same turn. Cutting Words and Font of Inspiration make sure foes always fail their saves.
Nature Cleric 1 - Can wear Heavy Armor on your mount without being slowed from a low STR, pick up Shillelagh and go Quarterstaff (build 9) with a capped WIS instead of STR. More spell slots. Channel Divinity at 2 just isn't worth it IMO.
War Cleric 2 - Heavy Armor. Channel Divinity +10 to hit can help Lance builds a lot. War Priest is also great for Lance builds, but doesn't synergize with Bestial Fury. War Priest will work pre-11 though. More spell slots.
Land Druid 2 - For Shillelagh Quarterstaff (build 9) with a capped WIS instead of STR. Wild Shape for utility. More spell slots. Refreshing spell slots. No metal armor...
Moon Druid 2 - Same as above but instead of refreshing spell slots you get more animal shapes. Meh.
Fighter 2/3 - Heavy Armor, Action Surge, Second Wind. Battle Master gives Maneuvers (most don't require the Attack action), and Superiority Dice. This is a strong option.
Paladin 2 - Heavy Armor. Smiting doesn't synergize with Bestial Fury, will help pre-11 though. Smiting doesn't work with ranged builds.
Assassin Rogue 3 - Assassinate doesn't synergize with Bestial Fury, will help pre-11 though.
Warlock 2/3 - Agonizing Blast always gives you a good ranged option. Pact of the Tome gets you another way to get Shillelagh if you want to do the Quarterstaff (build 9) with a capped WIS/CHR instead of STR. Refreshing spell slots.
None of these options stand out as optimal for DEX builds. Maybe Fighter? Ranger doesn't lose much from those last 2/3 levels anyway (just need 17 for 5th level slots, like Swift Quiver, Conjure Volley, etc). Still, Bestial Fury is just so strong that I'd get to 11 ASAP before multiclassing at all. At least as DEX. As STR, I can see how taking Barbarian and/or War Cleric dips early would be worth it. You're delaying Bestial Fury though, so keep that in mind.
VIII: Builds
Note: Added WIS mod from Foe Slayer when calculating level 20 builds (assumes Favored Enemy). Hunter's Mark up for all melee builds. Hunter's Mark or Swift Quiver, whichever is higher, for ranged builds. Assuming +1 weapon(s) by level 20. We are almost always taking Mounted Combatant to keep our pet/mount alive.
Note 2: This doesn't take into account any Multiclassing options.
(1) STR Lance and Shield build with Dueling style and a Panther. No mount until 3 means no Dueling bonus and no shield, so the first 2 levels will suck... At least you have d12 damage dice. ASIs: Shield Master > Mounted Combatant > STR*3. One Lance attack with mods and Hunter's Mark is going to out-damage 2 Panther attacks, so Bestial Fury isn't needed in this build. Wait...is that even true? 1d12+2+2+1d6 = ~14 damage from the Lance. ~6+4 + ~6+4 = ~20 damage from the Panther. What the hell. Bestial Fury, even with a crappy pet, beats PC Extra Attack, even with the best weapon available! Screw it, let's rely on Bestial Fury for damage from now on. Let's get a better pet though. Yeah, Pounce is nice, but if you really want to Prone (and frankly, you should try with a Lance), just Shield Bash shove instead. Let's throw this build in the fire and try the strongest pet we have, Giant Poisonous Snake, in the next build.
(2) STR Lance and Shield build with Dueling style and a Giant Poisonous Snake. You lose out on too much damage without Bestial Fury, so let's let the snake do what he does best: insane damage. You don't have Pounce for Prone anymore, but you still have Shield Bash to do it, so your Lance attacks won't be stuck with Disadvantage. Plus, the snake has a 10ft range, so it synergizes with Lance, letting you Bestial Fury from 10ft away. Always use your bonus action before your action, either to Hunter's Mark on round 1 or to Shield Bash for Prone when enemies run into 5ft range. I'd try to stay back at 10ft range though, because yeah, you can give your snake Advantage if the Prone succeeds, but the Lance's natural Disadvantage at 5ft range cancels out the Advantage that you'd get. Technically, 2 Advantage snake attacks if you succeed vs 1 Disadvantage Lance attack if you fail = 1 'net' Advantage attack, but I'm not a risk-taker. If you want to play this way though, the odds are in your favor to get into 5ft range. If you really want to get into 5ft range, just take a damn Longsword instead (see build 6). Damage at 11: ~13+4 + ~13+4 + 1d12+2+2+1d6 = ~48 damage. Damage at 20: ~13+6 + ~13+6 + 1d12+1+5+2+2+1d6 = ~58 damage. You have to wait until level 11, but this is significantly better than the Panther would have done. I'm just worried that you won't always land your Shield Bash. If you fail to Prone, you have Disadvantage, so the damage actually drops... Hard to calculate that though. And it sucks that you can't take Defensive Duelist, or this option would make an excellent tank.
(3) STR dual Lances with Two-Weapon Fighting style, Dual Wielder, and a Wolf. Suck until level 3 again. ASIs: Dual Wielder > Mounted Combatant > STR*3. Less reliable damage because enemies will often run into 5ft range. Wolf means there's still a chance to Prone, but not guaranteed, so some attacks might have Disadvantage at 5ft range. Try to stay back 10ft, but if an enemy is in 5ft range, then Bestial Fury means your Wolf will have 2 chances (both Attacks made with Advantage!) to knock the enemy prone. The biggest problem with dual Lances is that Two-Weapon Fighting requires you to take the Attack action, so it doesn't synergize with Bestial Fury. If you're Bestial Furying with a Wolf to attempt to prone, you can't take the bonus action to attack with the offhand Lance, so damage suffers. Calculating this damage is harder because sometimes you'll be in 10ft range and sometimes 5ft when an enemy approaches, so I'll calculate the average of the two. Damage at level 11 (10ft range): 1d12+2+1d6 + 1d12+2+1d6 + 1d12+2+1d6 = ~36 damage. Damage at level 11 (5ft range and Bestial Fury): ~7+4 + ~7+4 + 1d12+2+1d6 = ~34 damage. This damage is terrible and there's so much Disadvantage, I'm not even gonna bother finishing the math. Just no.
(4) STR dual Lances with Two-Weapon Fighting style, Dual Wielder, and a Panther. ASIs: Dual Wielder > Mounted Combatant > STR*3. Same as above but you take Panther for Pounce, which is slightly more reliable for Prone than a Wolf. You only have 1 chance at it though... Math is the same as above though (build 3), so low damage. These dual Lance builds are just terrible. If Bestial Fury synergized with Two-Weapon Fighting then maybe dual Lances would be great with a Giant Poisonous Snake. But it doesn't. And even then you'd have Disadvantage often...
(5) STR Longsword and Shield with Dueling style, Shield Master, and a Giant Poisonous Snake. Nice hybrid of Bestial Fury and Shield Bashing. Shield Master gives us something to do with our bonus actions (shove for Prone). If you want to be tankier, you can swap Dueling style for Defense style, but honestly, if your goal is to be an optimized tank, then we'll want to swap a lot more than just that (see the next build for a tanky version, build 6). ASIs: Shield Master > Mounted Combatant > STR*3. Damage at 11: ~13+4 + ~13+4 + 1d8+2+2+1d6= ~46 damage. Damage at 20: ~13+6 + ~13+6 + 1d8+1+5+2+2+1d6 = ~56 damage. It's actually okay. Like build (2) though, this isn't optimal because you lose out on Defensive Duelist.
(6) STR Shortsword and Shield with Defense style, Shield Master, Defensive Duelist, and a Giant Poisonous Snake. A tank variant to the above. You have to swap Longsword for Shortsword in order to take Defensive Duelist (Finesse required), and you lose a STR ASI too, so you're giving up some damage, but since most of your damage is coming from Bestial Fury anyway, I think it's worth it. This is also why I opt to drop Dueling style for Defense style. Your damage is already nerfed, so just focus on being a better tank. ASIs: Shield Master > Mounted Combatant > Defensive Duelist > STR*2. Damage at 11: ~13+4 + ~13+4 + 1d6+2+1d6 = ~43 damage. Damage at 20: ~13+6 + ~13+6 + 1d6+4+2+1d6 = ~51 damage. It's really not that bad of a damage drop, only 3-5 damage less than build (5), and you're suddenly much tankier. The Shield Bashing for Prone on bonus actions will give you and your allies a fair amount of Advantage as well.
(7) STR Longsword and no Shield with Dueling style and a Giant Poisonous Snake. This build makes use of the sword's Versatile feature to boost damage and frees up a feat to cap STR faster (no Shield means no Shield Master). Unfortunately, you can't take Defensive Duelist because Longsword isn't Finesse. Giant Poisonous Snake's Bestial Fury will put out the same damage as your Hunter's Marked d10 double Attacks and still give you an extra free attack, so it comes out ahead. ASIs: Mounted Combatant > STR*3 > WIS/another feat. Damage at 11: ~13+4 + ~13+4 + 1d10+3+2+1d6 = ~48 damage each round. Damage at 20: ~13+6 + ~13+6 + 1d10+1+5+3+2+1d6 = ~58 damage. Can go with Lucky instead of WIS to make up for no Advantage opportunities. This build suffers because you have nothing good to do with your bonus actions. Not a terrible option, but there are other builds with comparable damage that leave you much less vulnerable.
(8) STR dual Longswords with Two-Weapon Fighting, Dual Wielder, and a Giant Poisonous Snake. ASIs: Dual Wielder > Mounted Combatant > STR*3. The problem here is the same as before: Two-Weapon Fighting requires you to take the Attack action, so this doesn't synergize with Bestial Fury. It's either ~13+4 + ~13+4 = ~34 from a snake Bestial Fury or 1d8+2+6 + 1d8+2+6 = ~25. There's literally no point to taking a second Longsword over a Shield or going Versatile, because you're not out-damaging a Bestial Fury.
(9a) STR Quarterstaff and Shield with Dueling Style, Polearm Master, Mounted Combatant, and a Giant Crab. Okay...take a deep breath. Here we go. I considered making use of Versatile and dropping the Shield altogether, but the Dueling bonus only kicks in if you use the staff 1-handed. We could go with Defense fighting style instead of Dueling to make use of Versatile, but if you do the math: 2 AC from a Shield beats out 1 AC from the style, and 2 damage from Dueling beats out 1 average damage from the d8 vs d6. So keep the Shield and stick with Dueling style. Using a Shield means we can take Shield Master, but it's not ideal because we have competition for our bonus actions now: either Polearm Master's bonus attack or Shield Master's Shield Bash. So I dropped Shield Master. The original goal was to make use of Sentinel+Polearm Master to lock foes down at 10ft range for Bestial Fury (Giant Poisonous Snakes have 10ft range). Unfortunately, Quarterstaff only has 5ft range (despite being mentioned in Polearm Master). Also, when you Bestial Fury, you can't get the Polearm Master extra attack, so I had to make a choice: do more damage with the snake or focus on tanking and drop Bestial Fury. This build isn't going to out-damage the dedicated damage builds, so I chose to stop trying to max my damage and start to max my utility. Plus, Sentinel won't let you Bestial Fury for Opportunity Attacks, so we gotta get away from relying on our pet for damage. I dropped Sentinel because Quarterstaff means they'll be in 5ft range anyway... And I also dropped the snake and picked up a crab because it's tanky and can Grapple. I would still Bestial Fury now and then with the crab to Grapple some foes, but I'm not going to build around that. That's kind of just a bonus to this build. Instead, we rely on the extra Polearm Master attack that's boosted by Dueling style and Hunter's Mark. ASIs: Polearm Master > Mounted Combatant > STR*3. You can take Shield Master in place of a STR ASI if you really want extra tankiness, but I don't think it's needed. Hell, the crab's tanky enough that you might not even need Mounted Combatant! The only thing I'd be worried about are AOEs, which both feats help to mitigate. I'd still lean towards Mounted Combatant rather than Shield Master, just so you don't have the competing bonus actions as I mentioned above. The most beautiful thing about this build is that it reaches its mechanical potential once you get Extra Attack at level 5. Yes, level 5!!! Damage at 5: 1d6+2+2+1d6 + 1d6+2+2+1d6 + 1d4+2+2+1d6 = ~33 damage. Damage at 20: 1d6+1+5+2+2+1d6 + 1d6+1+5+2+1d6 + 1d4+1+5+2+1d6 = ~47 damage. Ehhh... The damage just isn't there. To be fair, you'll be getting a lot of extra damage from OAs, but you'd have to get an OA on every single round to catch up in damage to the other tank builds. At 5: ~33 + 1d6+2+2+1d6 = ~44 damage. At 20: ~47 + 1d6+1+5+2+1d6 = ~60 damage. Let's say you can OA 50% of the time and take an average. Average at 5: ~39 damage. Average at 20: ~54 damage. If you're confident you can pick up a bunch of OAs, then I guess this is an acceptable tanking option. It actually out-damages a few other builds by level 20! It all depends on how many OAs you can get.
EDIT: added build (9b)
(9b) STR Lance and Quarterstaff with Defense style, Sentinel, Polearm Master, and a Giant Crab. The problem with the build above (9a) was that Quarterstaff didn't have 10ft reach, so we couldn't make good use of Sentinel. Well, lucky for us, we can 1-hand both a Lance (while mounted, which we are) and a Quarterstaff. Now we can do the infamous Sentinel+Polearm lock-enemies-down-at-10ft OA combo because we have a Quarterstaff equipped and they enter our range at 10ft due to our Lance. It'd be so much easier if we could just use a Heavy polearm, like a Glaive or something, but nope. We're Small sized and we have to find crappy loopholes like this. Anyway, by locking enemies down 10ft away with our Lance, we don't have to worry about taking Mounted Combatant or Shield Master to protect our pet: only ranged attacks and AOEs will reach, and even when they do, it's a crab with high AC, so it's durable. By the way, we have to take Defense style, because Two-Weapon Fighting isn't useful for this build and Dueling requires only 1 weapon... Another downer is that we have to wait until 8 for the 2nd feat. Still faster than the Bestial Fury builds I suppose. ASIs: Polearm Master > Sentinel > STR*3. Half the time, you'll be doing damage with Lance at 10ft range and half the time they'll get through and you'll be doing damage with Quarterstaff+Polearm Master bonus action attack, so we'll count both and take an average. Damage at 8 (Lance): 1d12+2+1d6 + 1d12+2+1d6 = ~24 damage. Damage at 8 (Quarterstaff): 1d6+2+1d6 + 1d6+2+1d6 + 1d4+2+1d6 = ~26 damage. Average at 8: ~25 damage. Eww... Jebus, this build is awful. The Quarterstaff even out-damages the Lance! Yeah, you can finally do the Sentinel+Polearm Master+10ft range combo, but at what cost?! If you really wanna build a tank around this combo, don't pick a Small sized race.
(10) DEX Rapier and Shield with Dueling style and a Giant Crab. Anytime you don't see Giant Poisonous Snake, it's because we're trying to tank, not maximize damage. ASIs: Shield Master > Mounted Combatant > DEX*2 > WIS/another feat. Use first round bonus action to Hunter's Mark, then Shield Bash each round afterward to keep enemies Prone for constant Advantage. As we learned earlier, Bestial Fury will out-damage your Extra Attack, even with crappy pets. So just Shield Bash and follow with a Bestial Fury each round. Your damage isn't going to be great, but you're controlling the entire battlefield and tanking like a champ. Damage at level 11: ~5+4 + ~5+4 + 1d8+3+2+1d6 = ~31 damage each round. Damage at 20: ~5+6 + ~5+6 + 1d8+1+5+3+2+1d6 = ~41 damage. Really low damage, even for a tank... There are a lot of Grapples and Prones for Advantage, but I don't know if that's enough to make this build worth it...
(11) DEX Rapier and Shield with Dueling style, Defensive Duelist, and a Giant Poisonous Snake to make up for our lack of damage. Giant Poisonous Snake's Bestial Fury is powerful, combined with occasional Shield Bashing for Prone and Advantage, makes this build very strong. ASIs: Shield Master > Mounted Combatant > Defensive Duelist > DEX*2. Damage at 11: ~13+4 + ~13+4 + 1d8+3+2+1d6 = ~47 damage. Damage at 20: ~13+6 + ~13+6 + 1d8+1+5+2+2+1d6 = ~56 damage. This is really nice damage coming from a tank build! You should consider dropping Dueling Style for Defense style, since the snake is doing all the damn damage anyway.
(12) DEX dual Rapiers build with Two-Weapon Fighting style, Dual Wielder, and a Giant Poisonous Snake. Again, Two-Weapon Fighting doesn't stack with Bestial Fury, so this is out.
(13) DEX dual Rapiers build with Two-Weapon Fighting style, Dual Wielder, Defensive Duelist, and a Giant Poisonous Snake. Same as above (build 12). Nope.
(14) Ranged DEX Crossbow build with Archery style, Crossbow Expert and a Pteranodon. Bestial Fury and the bonus action attack from Crossbow Expert don't work together, so we're going for 3 Hand Crossbow attacks here: Attack, Extra Attack, bonus action Attack, all with Hunter's Mark. But wait, hold on, let's make sure it's worth giving up snake Bestial Fury before moving on with any of the ranged builds. So again, we want to compare the 2 snake bites you'd get from Bestial Fury to 2 crossbow attacks (Extra attack + bonus action attack) and see which comes out ahead. Hand Crossbow damage at 11: 1d6+3+6 + 1d6+3+6 = ~25 damage. Snake damage at 11: ~13+4 + ~13+4 = ~34 damage. 25 vs 34. Bestial Fury wins! But wait...that's only a 9 damage difference, which is less than a single Sharpshooter bonus (+10). So it looks like, if we can find a way to use at least 1 Sharpshooter effectively, then finally, something can beat out Bestial Fury! Archery style is giving us +2 already, so that makes the -5 from Sharpshooter 40% less bad. So, we can effectively Sharpshooter 40% more and still come out even. So let's do it. Technically, we're only using it 33% more when we add Sharpshooter to one of our three attacks, so the math is still in our favor. But wait, there's more! You can still Sharpshooter the free ranged attack you get from Bestial Fury! And if we're still doing it 33% of the time, that's another effective 3.3 damage added to Team Bestial Fury! So Bestial Fury barely pulls ahead again 35 vs 37! The reality is, the damage is so close either way... So is there a reason to lean one way or the other? Well, Bestial Fury requires you to put yourself in melee range. Without it, you can stand back and take less damage, so that's a perk. Plus, if you can somehow get Advantage, then you can put Sharpshooter on all 3 attacks instead of just 1 and actually pull far ahead of Bestial Fury in damage. That's another perk. So let's go with Sharpshooter instead of Bestial Fury. As a note, if enemies manage to get into 5ft range, you don't have to worry about Disadvantage because of Crossbow Expert. Since we're not relying on Bestial Fury at all in this build, we can drop the snake and pick up something more advantageous for ranged users. Try for a Pteranodon, otherwise a Giant Wolf Spider will do. ASIs: Crossbow Expert > Sharpshooter > Mounted Combatant > DEX*2. Damage at 8: 1d6+3+1d6+10 + 1d6+3+1d6 + 1d6+3+1d6 = ~40 damage. Damage at 20: 1d6+1+5+2+1d6+10 + 1d6+1+5+1d6 + 1d6+1+5+1d6 = ~51 damage. That's actually kinda low, so let's change a couple things for optimization. Let's go with Swift Quiver instead of Hunter's Mark. Sharpshooter 40% of the time averages +4 to each attack. Finally, let's switch to a Light Crossbow, since we're not using the bonus action attack once we gain access to Swift Quiver. Now, damage at 20: 1d8+1+5+2+4 + 1d8+1+5+4 + 1d8+1+5+4 + 1d8+1+5+4 = ~60 damage. This is an acceptable build, but not as good as the melee options. You reach your potential at 8 instead of 11 because you're not waiting for Bestial Fury, so there's that. If you can find a way to get Advantage (Faerie Fire, Inspiration, surprise, etc), then you can throw in a few more Sharpshots to boost this damage significantly. Sharpshooter on all 4 shots would bring the average damage at 20 up to ~84! Ranged builds are just super reliant on Sharpshooter to compete with Melee builds.
(15) Ranged DEX Crossbow build with Archery style, Crossbow Expert, Lucky and a Pteranodon. Variant to above but drop Mounted Combatant for Lucky. Giant Wolf Spider if you can't get a Pteranodon. The main reason we're taking Lucky is to make our Sharpshots more reliable, but it should be noted that it will also help us when we really need to land a skill check or save. Mounted Combatant isn't a priority because you're actively avoiding melee range anyway... You could, theoretically, take both Lucky AND Mounted Combatant, and drop a DEX ASI. But since we're relying so much on our own damage and need to be more accurate to land our Sharpshots, let's just go with 2 DEX > Mounted Combatant (I can't tell you how often I was just 1 point off from an enemy's AC). Without Mounted Combatant, our pet still boosts our speed and potentially give us flight or wall-climbing, so we still benefit from being Small and riding it. Worst case scenario though, we'll still be just as effective with this build if our pet dies. Sharpshooter lets us attack from long range without Disadvantage, so just run to 120ft with a Hand crossbow or 320ft with a Light Crossbow and drop Mounted Combatant altogether. ASIs: Crossbow Expert > Sharpshooter > Lucky > DEX*2. Damage at 8: 1d6+3+1d6+10 + 1d6+3+1d6 + 1d6+3+1d6 = ~40 damage. Same swaps as above build (14) at level 20: Light Crossbow, Swift Quiver, and 40% Sharpshot instead of 33%. Damage at 20: 1d8+1+5+2+4 + 1d8+1+5+4 + 1d8+1+5+4 + 1d8+1+5+4 = ~60 damage. Literally, the same damage as above (build 14), but more reliable because of Lucky. No biggie if your pet dies, but stay the hell back anyway. The point of going Small is to ride it in the first place, so at least try to keep it alive. After all, if you're not planning to ride your pet, why are you limiting yourself to Small races? And like above, the more Sharpshots you can squeeze in, the better, so looks for other (non-Lucky) ways to get Advantage as well. Firing from long range (yay Sharpshooter) can often give you Advantage on a surprise round. Again, 4 Sharpshots would bring average damage at 20 all the way up to ~84 each round.
(16) Ranged DEX Crossbow build with Archery style, Crossbow Expert, Sharpshooter, Lucky, and a Giant Poisonous Snake. Let's see what the damage would be with snake Bestial Furies. Start with a Light Crossbow from levels 1-3 for the larger d8 damage. At level 3, your snake will out-damage you, so just command it to attack instead of you. At level 4, you pick up Crossbow Expert and should switch to a Hand Crossbow so you can bonus action attack all the way to 10. Alternatively, you can command your pet to Attack and then follow with a free attack, but then you lose your bonus action attack. I'd stick to Extra Attack during these levels. Light Crossbow from 11-20 again because you're going to Bestial Fury and can't bonus action attack anyway. Swift quiver at 17 gives this a big boost, especially with a Light Crossbow instead of Hand Crossbow. You still need Crossbow Expert to remove Loading for Swift Quiver to work, but we've had that feat since level 4. You can Bestial Fury from 10ft range (snake has 10ft reach), but even if you have to get into 5ft range, Crossbow Expert removes Disadvantage, so just ride the snake the entire time. Unfortunately, you can't do anything with your bonus actions from 11-16, but that's okay, because Bestial Fury makes up for the damage. With Swift Quiver up, you get off a snake Bestial Fury AND 3 Light Crossbow attacks! Like I showed in build (15), the math is in your favor to use Sharpshooter on that free Bestial Fury shot 40% of the time. In fact, I'm gonna add +4 (40% of Sharpshooter's +10) when calculating damage at level 11, and add +10 (for one use of Sharpshooter per round) when you Swift Quiver 17+. You could drop a DEX ASI for Lucky. I wouldn't drop Mounted Combatant though since your pet needs to get close to enemies for Bestial Fury, and you can ride it because Crossbow Expert lets you shoot from 5ft. Most of the damage from the snake at this point too, so dropping 1 DEX for ensuring you'll land a few Sharpshots is worth it. Let's go with Lucky right after Sharpshooter, and take Mounted Combatant early so you can sit comfortably in melee range and keep your pet safe. ASIs: Crossbow Expert > Mounted Combatant > Sharpshooter > Lucky > DEX. Damage at 12 (gotta wait til 12 for Sharpshooter): ~13+4 + ~13+4 + 1d8+3+1d6+4 = ~49 damage. Damage at 20: ~13+6 + ~13+6 + 1d8+1+4+2+10 + 1d8+1+4 + 1d8+1+4 = ~79 damage. Whoa. It's too bad that you can't Bestial Fury and still get your bonus action Hand Crossbow attack in levels 11-16 or the damage would be even higher for those midrange levels. Also, if you're relying on Bestial Fury, your pet has to be in close range, and you can't protect it with Mounted Combatant until level 8... That leaves your pet vulnerable for 5 levels of the game (you're not gonna be putting out these high numbers if your pet keeps dying, right?). If you're starting high level, this is the best build out there, but if you're not, just be mindful of the fact that your pet will be somewhat vulnerable for a few levels. Getting Sharpshooter on all 3 bolts when you get Advantage will boost damage by another 12, for ~91 damage a round!
(17) Ranged DEX Light Crossbow with Crossbow Expert, Sharpshooter, Lucky, and a Pteranodon. This build focuses on using the bonus action pet Help to give you Advantage so you can Sharpshooter more often. Flyby Help with a Pteranodon, or if DM won't allow it, then send a Giant Crab in alone. You could ride a crab into 5ft range because of Crossbow Expert negating Disadvantage, but this build is very feat-heavy as is, so we don't get access to Mounted Combatant until very late. Might as well just not ride the pet so we don't have to take Mounted Combatant at all (we really need the DEX since we're using Sharpshooter more often than the other builds). Sharpshooter 100% of the time on the first attack (because of Advantage) and 40% of the time on the Extra Attack (so +4 when doing math). It's a more reliable build than the ones above, but you're going to want to Swift Quiver at 17+, so no more Advantage/reliability. You also lose the Hunter's Mark boost at 20. ASIs: Crossbow Expert > Sharpshooter > Lucky > DEX*2. Damage at 8: 1d8+3+1d6+10 + 1d8+3+1d6+4 = ~36 damage. Damage at 20: 1d8+1+5+2+4 + 1d8+1+5+4 + 1d8+1+5+4 + 1d8+1+5+4 = ~56 damage. By 20, this catches up to the the other builds, but this build falls behind too much for most of the game.
(18) Ranged DEX with Shortbow and Archery style, Sharpshooter, Lucky, and a Pteranodon. The good news about using a bow is that we can drop the Crossbow Expert feat. Unfortunately, we don't have access to the Heavy, better Longbow. We also lose out on Crossbow Expert's bonus action attack, so if this build is gonna be good, then Sharpshooter needs to land. This build relies on the Pet Help bonus action to give us Advantage enough to proc Sharpshooter often. The Lucky feat will help us out a lot too. Unfortunately, our pet has to get into 5ft range for the Help action, so Pteranodon can flyby or a Giant Crab can go in safely alone (don't ride it). This build doesn't require a lot of feats, so we can cap DEX faster. Mounted Combatant isn't necessary because we're not riding our pet if it isn't a Pteranodon, and even then, you can just fly out of range of danger most of the time. Sharpshooter 100% of the time on the first attack (because of Advantage) and 40% of the time on the Extra Attack (so +4 when doing math). Swift Quiver as your bonus action will out-perform Advantage on the first attack, so 17+ we stop making our pet use the Help action. This build is not spectacular, but it caps mechanically really early, at level 7, or even level 5 if you don't want to Sharpshooter 100% of the time on the first attack. ASIs: Sharpshooter > Lucky > DEX*2 > WIS/something else. Damage at 7: 1d6+3+1d6+10 + 1d6+3+1d6+4 = ~34 damage. Eww. Damage at 20: 1d6+1+5+3+4 + 1d6+1+5+4 + 1d6+1+5+4 + 1d6+1+5+4 = ~57 damage. Meh. Okay by 20, but might as well go Crossbow.
(19) Ranged DEX with Shortbow, Sharpshooter, Lucky, Crossbow Expert, and a Giant Poisonous Snake. Instead of relying on Extra Attack with build (18) above, we rely on Bestial Fury for bigger damage. The problem again though, is that you don't have Crossbow Expert for negating Disadvantage at 5ft range. So you can't ride your snake and protect it with Mounted Combatant. I mean, you could start out at 10ft range and Bestial Fury because the snake has reach, but then when enemies run into 5ft range, you're boned. Yes, your snake will do a lot of damage on its own (way more than you with a crappy Shortbow), but having Disadvantage half the time is bad. Might as well go dual Lances for the higher hit dice (build 4) if you don't mind the Disadvantage. There are two options: 1) take Crossbow Expert literally for the 5ft range aspect so you can ride your snake and keep it alive, or 2) send in your snake alone with no way to protect it. Remember folks: dead pets do ~0 average damage. So let's take Crossbow Expert (ugh) and see what numbers we can pump out. And for the record, your snake is gonna out-damage you early on, so at level 5 when you get Extra Attack, you should command it to attack and get a free bow shot in afterward. And if we're doing that, then the snake has to be in melee range early on, so might as well take Crossbow Expert early to negate that occasional Disadvantage (double ugh). You're gonna need Mounted Combatant early too (triple ugh)... ASIs: Crossbow Expert (yes, take it first...) > Mounted Combatant > Sharpshooter > Lucky > DEX. Yep, you can't cap your DEX anymore. You could drop Lucky for DEX, but I wouldn't recommend it. Better you land Sharpshooter 40% of the time and rely on your pet for most of the attacks. Damage at 12 (gotta wait for 12 to get Sharpshooter): ~13+4 + ~13+4 + 1d6+3+1d6+4 = ~48 damage. Damage at 20 (with Swift Quiver 33%): ~13+6 + ~13+6 + 1d6+1+4+2+10 + 1d6+1+4 + 1d6+1+4 = ~76 damage. What the hell? Damn, snake, you scary. Crossbow still comes out a whopping 1-3 damage ahead, but this is a surprisingly good alternative.
(20) DEX Hybrid Rapier and Hand Crossbow with Two-Weapon Fighting style, Crossbow Expert, and a Giant Poisonous Snake. Best of both worlds and versatility on the battlefield. Crossbow Expert gives you that extra bonus action attack, even if your main hand weapon isn't Light. Make sure your DM is cool with RAW so you don't need a free hand to reload the Hand Crossbow (Crossbow Expert RAW removes Loading property). This build is still held back by the fact that Bestial Fury doesn't synergize with Crossbow Expert (requires Attack action to get the bonus action attack). Two Rapier swings ain't gonna out-damage your snake. Still, having the option to melee or ranged when needed is nice. Without that extra bonus action attack, I can already tell you that it's not going to out-damage the Light Crossbow build (14), and frankly, Crossbow Expert's 5ft range feature makes this hybrid concept pointless if you can't get a better mainhand weapon. Not even gonna do the math.
(21) STR/DEX Hybrid Lance and Hand Crossbow with Two-Weapon Fighting style, Crossbow Expert, and a Giant Poisonous Snake. Let's try this with a stronger mainhand weapon. Kinda MAD. Bestial Fury doesn't synergize with Crossbow Expert, so the question becomes: do we have to rely on Bestial Fury? Do 2 snake bites out-damage 1 Lance attack + 1 bonus action Hand Crossbow attack? ASIs: Crossbow Expert > Mounted Combatant (tempted to drop this but it's just so good) > DEX > STR > DEX. You'll be using DEX more than STR because you'll usually be stuck in 5ft range. At level 11, 2 snake bites: ~13+4 + ~13+4 = 34 damage. At level 11, 1 Lance attack + 1 bonus action Hand Crossbow with Hunter's mark does: 1d12+2+6 + 1d6+3+6 = ~27 damage. The snake wins! By 20, you catch up a bit: 1d12+1+3+2+6 + 1d6+1+5+6 = ~34 damage. Oh wait, your snake get another +2*2 from your higher proficiency bonus by 20, so actually the snake still wins. Plus, like I said earlier, you'll probably be in 5ft range a bunch, so your damage goes down even more if you can't Lance.
IX. Conclusion
So what did we learn? Giant Poisonous Snakes put out a lot of damage, so Bestial Fury builds tend to shine. Unfortunately, a lot of our options, such as Two-Weapon Fighting's bonus attack and Crossbow Expert's bonus attack, don't synergize with Bestial Fury.
The early feat requirements of the STR builds leaves you with a 14 STR for 11 levels, making the Bestial Fury all the more important. Your Small size also severely limits your weapon choices. If you're going STR, Lance and Shield (build 2) is the best Lance build, but missing a Shield Bash shove will punish you with Disadvantage. I would just avoid Lances altogether. Dual Lances doesn't work with Bestial Fury, so no help there. The Longsword builds don't let you take Defensive Duelist, so even when using Versatile, you are taking too much damage to be optimal. Shortsword and Shield (6) will be more reliable and lets you tank well. The Quarterstaff Sentinel build (9a) is actually a great tanking option as well, but relies heavily on Opportunity Attacks to make its damage acceptable.
If you're going DEX, congratulations, you have more wiggle room. The racial DEX boost opens up extra feats and maximizes damage faster. Dual Rapiers is out, but the Rapier and Shield with Defensive Duelist (11) puts out great numbers while still being tanky. It ends up being slightly more optimal than build (6) above.
Your Small size prevents you from using Heavy Crossbows and Longbows, so the ranged options suffer. The Crossbow Expert feat's wording prevents synergy with Bestial Fury, so that messes things up a bit too. Sharpshooter is amazing though, and can keep up with Bestial Fury damage if you throw it on more than 1 attack per round. The bonus action pet Help builds are meh. The best numbers come from using Crossbows (alternate between Light Crossbow and Hand Crossbow) and relying on snake Bestial Fury for damage (16). Shortbow (19) can almost keep up, so don't dismiss it, but if we're going for optimal then stick to Crossbow.
Hybrid melee+ranged builds just suck.
tl;dr
DEX>STR. Ranged>Melee. Bestial Fury>Extra attack. 2+ Sharpshooter>Bestial Fury>1 Sharpshooter. Lance is a trap (gotta worry about Disadvantage too much). Build (9a) is best for straight tank (great control), build (11) is best for general melee (balances good damage with tankability), and build (16) is the best ranged (balances high damage with survivability). Overall, build (16) puts out the biggest numbers without succumbing to glass cannon-ism, so that's technically the most optimal.
Small size Beastmaster
Why Small size? Because you can ride your Animal Companion and take the Mounted Combatant feat. That doesn't sound like much, but it's actually quite significant. Pets have relatively low HP (4HP per Beastmaster level) and a crap AC (until you invest in Barding way down the line), so Mounted Combatant will keep it alive much longer. It also gives you a boost to speed and could give you (if the DM allows it) access to flight right at level 3.
I. Races
Gnome or Halfling.
Gnomes get better racials IMO. Rock Gnomes don't do much for you, but Forest Gnomes fit thematically and give you a lot of goodies. You get +1 DEX, you can talk to small animals, you get the Minor Illusion cantrip for free (this is awesome), and you get Advantage on INT, WIS, and CHR saves vs Magic (this is even more awesome). Deep Gnomes are an okay option too, but only with the feat they get. Since the builds below are somewhat feat-heavy, you can't take the optional magic feat, making Deep Gnomes less optimal.
Halflings get +2 DEX, which is amazing. But point buy only guys up to 15, so you still can't hit 18 to get +4. Yeah, it's 1 DEX more than Forest Gnomes, but for that you give up a lot. Their Lucky trait isn't as good as the feat, but it's still great because it will prevent Natural 1s (honestly, those kinda make the game fun though...). If you have to go Halfling, I'd go Stout for the CON and poison resistance. Lightfoot doesn't give you anything worthwhile.
Forest Gnome looks to be the most optimal if you're going point buy. Stout Halfling and Forest Gnome are both optimal choices if you can roll your stats and break the +3 mod barrier. I'd still lean towards Forest Gnome though, because the racials are just that much better. If you roll, and your highest number is even (14, 16, or 18), then I'd consider the Stout Halfling better at that point because it'll potentially open up a feat later on.
II. STR vs DEX
Point buy STR: STR 14, DEX 14 (13+1), CON 12, INT 12 (10+2), WIS 14, CHR 10
Point buy DEX: STR 8, DEX 16 (15+1), CON 14, INT 12 (10+2), WIS 14, CHR 10
DEX comes out ahead.
III. Weapons
Somehow, Small PCs are capable of carrying Lances. For STR, Lance and Shield sounds good. Highest damage (d12) and still boost AC, right? This requires the Shield Master feat to be good though, because without shoving a foe back to 10ft range or Prone, you're gonna have Disadvantage a lot... There are some pets that can Prone though too, so we'll have to see. Your Small size means no Heavy weapons, so you're limited to weapons like Longsword (d8 or d10 versatile). Not as optimal as the Lance, but it doesn't require a feat. If you want to take the Dual Wielder feat, two Lances (d12*2) would be hilarious. Unfortunately, without the Shield Master shove combo, you're going to have Disadvantage pretty often with that too. Two Longswords (d8*2) would be more reliable. Apparently Bestial Fury doesn't work with Two-Weapon Fighting though, so you can't do both. I'll do the math later in the Builds section.
My first instinct with DEX is to go Ranged, but again, our Small size means no Heavy weapons like Longbow or Heavy Crossbow. Fighting Style gives you a few good options. Archery style Light Crossbow's damage (d8) is equivalent to a Rapier until Crossbow Expert is picked up at level 4, giving you 2 Hand Crossbows (d6*2). Two Weapon Fighting style Rapiers (d8*2) looks attractive, but again, can't Bestial Fury with them. Dueling style with a Rapier (d8) and Shield if you really want the AC, but damage doesn't look as good as the others. It might work if your Bestial Furies come out ahead of Extra Attack and a bonus action Attack though. Again, I'll do the math below.
IV. Feats
Mounted Combatant looks necessary regardless of your build. Your Small size and Medium mount nerfs the Advantage on attacks vs smaller enemies feature, but the ability to "tank" for your low HP pet is crucial regardless. This will help significantly at higher levels, since your pet's HP scales poorly (40HP at level 10 means a single Dragon's Breath will 1-shot it)... Ranged characters will take less attacks though, so maybe it doesn't have to be rushed for ranged builds.
Shield Master is a must if you're going Lance and Shield. Lance has Disadvantage at 5ft range. Shield Master lets you shove enemies either Prone or into 10ft range as a bonus action before attacking them with the Lance, negating the Disadvantage. Note: Prone won't give your Once you get Bestial Fury, you have to start shoving Prone instead of knocking them back because the Bestial Fury requires you to be in 5ft range. The only good exception to this is Giant Poisonous Snake, because it has a 10ft range. We'll go more into that later though.
Crossbow Expert looks interesting. If you really want a ranged character then this looks really good. You can free-load the same Hand Crossbow twice to attack with both action and bonus action, and still hold a Shield for the extra AC. (Common sense says you would need a free hand, but this feat specifically negates Loading aspect of crossbows so it's RAW!) Unfortunately, RAW doesn't give you the bonus action attack unless you take the Attack action, so you can't Bestial Fury and still get the extra attack. That's a big deal when you see just how much damage some pets can put out by themselves.
Sharpshooter will boost the crap out of your ranged damage. More useful when you can get Advantage. At level 7, Rangers can command their pet to Help for Advantage as a bonus action. I smell synergy.
Dual Wielder gives you 2 Rapiers (d8*2) and still boosts your AC for melee DEX users. For STR builds, this opens up Double Longswords (d8*2). As I said before though, dual Lances (d12*2) sounds amazing, but you will have Disadvantage often without the Shield Master shove combo. Also, Two-Weapon Fighting requires you to take the Attack action, so this doesn't synergize with Bestial Fury, eliminating a LOT of good build options...
Defensive Duelist could be nice for certain builds. Makes you really tanky even without a Shield, but requires a Finesse Weapon. Opens up extra non-Shield build options, because you can still tank for your pet with Mounted Combatant and this feat.
Lucky is good for everyone. Great for Lance builds that need the Advantage as well as ranged builds that want Advantage for Sharpshooter.
Sentinel and Polearm Master synergize well, and you can try a Quarterstaff build around this. Giant Poisonous Snakes even have a 10ft reach, so you can Bestial Fury with them at 10ft range. Unfortunately, to get the damage boost from Polearm master, you have to take the Attack Action rather than Bestial Fury, so this is a mess. Fortunately, Hunter's Mark kicks in on that extra attack and makes it worth it. Kinda. I'll dedicate a build below to this to see what's up.
V: Pet
Ask your DM if you can use the Animal Friendship spell to "upgrade" your Animal Companion to something strong you run into in your travels with a higher CR than 1/4. The answer will probably be no, but it can't hurt to ask.
Note: You get to add your Proficiency bonus to pet attacks, but the damage listed in this section doesn't include that. See the Builds section for complete math.
Melee options:
Giant Badger - 1d6+1 = ~4 damage. Looks weak at first, but two things can change that significantly: 1) if your DM allows Multiattack (RAW), and 2) if you get to add your Proficiency bonus to each attack of that Multiattack (unclear). If the DM is cool with both of these things, then the badger will out-damage other pets and should replace the snake below in Bestial Fury builds (that don't rely on the snake's 10ft range). I'm assuming a lot of DMs won't allow proficiency stacking, so I won't even bother making builds around this pet. Can't hurt to ask your DM though!
Giant Poisonous Snake - 1d4+4+3d6(/2) = ~17(12) damage. Holy hell. This is twice as good as some of the other pet options. The DC 11 CON save is easy to make though. We'll say any roll of 5 or less on a d20 fails the save, so when doing the math, we can say there's a 25% chance to fail the save. When you do the math, (12+12+12+17)/4 = 13.25. For sake of ease, we'll just say the snake averages ~13 damage per attack. Also, with +6 to hit, it's very accurate! This pet is the bread and butter of Bestial Fury builds. Oh, and did I mention it has a 10ft reach? 10ft range synergizes with Lance, Quarterstaff+Sentinel, and Ranged builds.
Panther - 1d6+2 = ~6 damage. - Pounce is nice because Prone. Prone looks particularly good for Lance users because the Advantage on a Prone enemy negates the Disadvantage from being in 5ft range, allowing you to get your pet in range for Bestial Fury. It's kind of a trap though. The Panther damage is crap compared to your Lance attacks, especially with modifiers and Hunter's Mark. Pouncing does potentially increase damage on the first round (1d6+2 + 1d4+2 = ~10 damage), so Bestial Fury Pounce for that initial Prone is actually an effective tactic. I wouldn't Bestial Fury after that though.
Wolf - 2d4+2 = ~7 damage. Pack Tactics gives it Advantage 100% of the time while you ride it, so despite lower damage, it's actually an okay option for Bestial Fury builds. Speed 40, Stealth bonus, free chance to Prone. Not optimal most of the time, but this is a solid backup choice at all levels.
Ranged options:
Pteranodon - 2d4+1 = ~6 damage. Flight. Best option for Ranged users because there won't always be walls to climb when you want to get away.
Giant Wolf Spider - 1d6+1+2d6(/2) = ~12(8) damage. Same 25% poison chance as with the snake. (8+8+8+12)/4 = ~9 damage average. That's pretty darn good! Wall climbing, Stealth bonus, speed 40, poison. Good for ranged builds. Another solid Bestial Fury backup, though personally, I like the Advantage from Pack Tactics more.
Other options:
Giant Frog - 1d6+1 = ~5 damage. Free Grapples. Swallowing is hilarious. Optimal choice for fun?
Giant Crab - 1d6+1 = ~5 damage. Naturally high AC, but Barding and Mounted Combatant make that less useful. You can safely send it in alone for ranged builds to Help you for Advantage when Sharpshooting. The Grapple feature makes this a great option for tank builds too.
tl;dr It's hard to pick the best pet because it's so campaign-dependent and every build is different. Looks like Giant Poisonous Snake would amount in the most damage for melee builds that want to use Bestial Fury. Panther and Wolf are tactical for Lance users because of Prone. Giant Crab is good for tank builds that don't need high damage. Pteranodon if DM allows flight, else Giant Wolf Spider for kiting ranged builds. Giant Crab for ranged builds where you send your pet in alone to Help (don't ride it). Giant Poisonous Snake for ranged builds that still want to use Bestial Fury (don't ride it).
VI. Spells
Cure Wounds/Goodberry for heals. A heal spell is always good as a backup.
Hunter's Mark is great as a first round spell, doesn't synergize well with Bestial Fury (because you're losing Extra Attack), but lasts the entire encounter. Unless you're riding a Pteranodon, I don't recommend casting it with a higher level slot, even though it could potentially last all day. Why? Because it requires Concentration, and you're getting hit or tanking for your pet often enough if you aren't ranged.
Pass Without Trace is amazing for you, your pet, and your teammates. Take it.
Stoneskin, especially at 15 when it also works on your pet, can be good for Bestial Fury tank builds. Losing Hunter's Mark for this isn't a big deal when most of your damage is coming from your pet anyway.
Swift Quiver is good for some builds and bad for others. First of all, you don't get it until level 17. Frankly, you should never build a character around mechanics that don't kick in for the first 16 levels of your game. So if you're starting below level 10, pretend this spell doesn't exist, because chances are you'll never see it in action anyway. Crossbow Expert already gives us a free bonus action attack, but we won't use that bonus action attack because Bestial Fury doesn't synergize with it. So comboing with Bestial Fury is a good use of this spell. It combos well with Sharpshooter too, especially vs low AC foes. Unfortunately, not a lot of enemies will have a low AC at level 17... So yeah, it depends on if your build makes use of Bestial Fury and/or Sharpshooter.
VII. Multiclassing
Barbarian 1/2 - Rage synergizes with Bestial Fury (doesn't require Attack action). Only for STR builds. Reckless Attack would work well for Lance builds.
Lore Bard 3 - Cutting Words on saves like Poison and Grapple. More spell slots.
Lore Bard 6 - Lmfao. Additional Magical Secrets for Enlarge or Haste. Enlarge to make a Giant Giant Frog to Swallow Medium size foes or Haste it to Grapple and Swallow on the same turn. Cutting Words and Font of Inspiration make sure foes always fail their saves.
Nature Cleric 1 - Can wear Heavy Armor on your mount without being slowed from a low STR, pick up Shillelagh and go Quarterstaff (build 9) with a capped WIS instead of STR. More spell slots. Channel Divinity at 2 just isn't worth it IMO.
War Cleric 2 - Heavy Armor. Channel Divinity +10 to hit can help Lance builds a lot. War Priest is also great for Lance builds, but doesn't synergize with Bestial Fury. War Priest will work pre-11 though. More spell slots.
Land Druid 2 - For Shillelagh Quarterstaff (build 9) with a capped WIS instead of STR. Wild Shape for utility. More spell slots. Refreshing spell slots. No metal armor...
Moon Druid 2 - Same as above but instead of refreshing spell slots you get more animal shapes. Meh.
Fighter 2/3 - Heavy Armor, Action Surge, Second Wind. Battle Master gives Maneuvers (most don't require the Attack action), and Superiority Dice. This is a strong option.
Paladin 2 - Heavy Armor. Smiting doesn't synergize with Bestial Fury, will help pre-11 though. Smiting doesn't work with ranged builds.
Assassin Rogue 3 - Assassinate doesn't synergize with Bestial Fury, will help pre-11 though.
Warlock 2/3 - Agonizing Blast always gives you a good ranged option. Pact of the Tome gets you another way to get Shillelagh if you want to do the Quarterstaff (build 9) with a capped WIS/CHR instead of STR. Refreshing spell slots.
None of these options stand out as optimal for DEX builds. Maybe Fighter? Ranger doesn't lose much from those last 2/3 levels anyway (just need 17 for 5th level slots, like Swift Quiver, Conjure Volley, etc). Still, Bestial Fury is just so strong that I'd get to 11 ASAP before multiclassing at all. At least as DEX. As STR, I can see how taking Barbarian and/or War Cleric dips early would be worth it. You're delaying Bestial Fury though, so keep that in mind.
VIII: Builds
Note: Added WIS mod from Foe Slayer when calculating level 20 builds (assumes Favored Enemy). Hunter's Mark up for all melee builds. Hunter's Mark or Swift Quiver, whichever is higher, for ranged builds. Assuming +1 weapon(s) by level 20. We are almost always taking Mounted Combatant to keep our pet/mount alive.
Note 2: This doesn't take into account any Multiclassing options.
(1) STR Lance and Shield build with Dueling style and a Panther. No mount until 3 means no Dueling bonus and no shield, so the first 2 levels will suck... At least you have d12 damage dice. ASIs: Shield Master > Mounted Combatant > STR*3. One Lance attack with mods and Hunter's Mark is going to out-damage 2 Panther attacks, so Bestial Fury isn't needed in this build. Wait...is that even true? 1d12+2+2+1d6 = ~14 damage from the Lance. ~6+4 + ~6+4 = ~20 damage from the Panther. What the hell. Bestial Fury, even with a crappy pet, beats PC Extra Attack, even with the best weapon available! Screw it, let's rely on Bestial Fury for damage from now on. Let's get a better pet though. Yeah, Pounce is nice, but if you really want to Prone (and frankly, you should try with a Lance), just Shield Bash shove instead. Let's throw this build in the fire and try the strongest pet we have, Giant Poisonous Snake, in the next build.
(2) STR Lance and Shield build with Dueling style and a Giant Poisonous Snake. You lose out on too much damage without Bestial Fury, so let's let the snake do what he does best: insane damage. You don't have Pounce for Prone anymore, but you still have Shield Bash to do it, so your Lance attacks won't be stuck with Disadvantage. Plus, the snake has a 10ft range, so it synergizes with Lance, letting you Bestial Fury from 10ft away. Always use your bonus action before your action, either to Hunter's Mark on round 1 or to Shield Bash for Prone when enemies run into 5ft range. I'd try to stay back at 10ft range though, because yeah, you can give your snake Advantage if the Prone succeeds, but the Lance's natural Disadvantage at 5ft range cancels out the Advantage that you'd get. Technically, 2 Advantage snake attacks if you succeed vs 1 Disadvantage Lance attack if you fail = 1 'net' Advantage attack, but I'm not a risk-taker. If you want to play this way though, the odds are in your favor to get into 5ft range. If you really want to get into 5ft range, just take a damn Longsword instead (see build 6). Damage at 11: ~13+4 + ~13+4 + 1d12+2+2+1d6 = ~48 damage. Damage at 20: ~13+6 + ~13+6 + 1d12+1+5+2+2+1d6 = ~58 damage. You have to wait until level 11, but this is significantly better than the Panther would have done. I'm just worried that you won't always land your Shield Bash. If you fail to Prone, you have Disadvantage, so the damage actually drops... Hard to calculate that though. And it sucks that you can't take Defensive Duelist, or this option would make an excellent tank.
(3) STR dual Lances with Two-Weapon Fighting style, Dual Wielder, and a Wolf. Suck until level 3 again. ASIs: Dual Wielder > Mounted Combatant > STR*3. Less reliable damage because enemies will often run into 5ft range. Wolf means there's still a chance to Prone, but not guaranteed, so some attacks might have Disadvantage at 5ft range. Try to stay back 10ft, but if an enemy is in 5ft range, then Bestial Fury means your Wolf will have 2 chances (both Attacks made with Advantage!) to knock the enemy prone. The biggest problem with dual Lances is that Two-Weapon Fighting requires you to take the Attack action, so it doesn't synergize with Bestial Fury. If you're Bestial Furying with a Wolf to attempt to prone, you can't take the bonus action to attack with the offhand Lance, so damage suffers. Calculating this damage is harder because sometimes you'll be in 10ft range and sometimes 5ft when an enemy approaches, so I'll calculate the average of the two. Damage at level 11 (10ft range): 1d12+2+1d6 + 1d12+2+1d6 + 1d12+2+1d6 = ~36 damage. Damage at level 11 (5ft range and Bestial Fury): ~7+4 + ~7+4 + 1d12+2+1d6 = ~34 damage. This damage is terrible and there's so much Disadvantage, I'm not even gonna bother finishing the math. Just no.
(4) STR dual Lances with Two-Weapon Fighting style, Dual Wielder, and a Panther. ASIs: Dual Wielder > Mounted Combatant > STR*3. Same as above but you take Panther for Pounce, which is slightly more reliable for Prone than a Wolf. You only have 1 chance at it though... Math is the same as above though (build 3), so low damage. These dual Lance builds are just terrible. If Bestial Fury synergized with Two-Weapon Fighting then maybe dual Lances would be great with a Giant Poisonous Snake. But it doesn't. And even then you'd have Disadvantage often...
(5) STR Longsword and Shield with Dueling style, Shield Master, and a Giant Poisonous Snake. Nice hybrid of Bestial Fury and Shield Bashing. Shield Master gives us something to do with our bonus actions (shove for Prone). If you want to be tankier, you can swap Dueling style for Defense style, but honestly, if your goal is to be an optimized tank, then we'll want to swap a lot more than just that (see the next build for a tanky version, build 6). ASIs: Shield Master > Mounted Combatant > STR*3. Damage at 11: ~13+4 + ~13+4 + 1d8+2+2+1d6= ~46 damage. Damage at 20: ~13+6 + ~13+6 + 1d8+1+5+2+2+1d6 = ~56 damage. It's actually okay. Like build (2) though, this isn't optimal because you lose out on Defensive Duelist.
(6) STR Shortsword and Shield with Defense style, Shield Master, Defensive Duelist, and a Giant Poisonous Snake. A tank variant to the above. You have to swap Longsword for Shortsword in order to take Defensive Duelist (Finesse required), and you lose a STR ASI too, so you're giving up some damage, but since most of your damage is coming from Bestial Fury anyway, I think it's worth it. This is also why I opt to drop Dueling style for Defense style. Your damage is already nerfed, so just focus on being a better tank. ASIs: Shield Master > Mounted Combatant > Defensive Duelist > STR*2. Damage at 11: ~13+4 + ~13+4 + 1d6+2+1d6 = ~43 damage. Damage at 20: ~13+6 + ~13+6 + 1d6+4+2+1d6 = ~51 damage. It's really not that bad of a damage drop, only 3-5 damage less than build (5), and you're suddenly much tankier. The Shield Bashing for Prone on bonus actions will give you and your allies a fair amount of Advantage as well.
(7) STR Longsword and no Shield with Dueling style and a Giant Poisonous Snake. This build makes use of the sword's Versatile feature to boost damage and frees up a feat to cap STR faster (no Shield means no Shield Master). Unfortunately, you can't take Defensive Duelist because Longsword isn't Finesse. Giant Poisonous Snake's Bestial Fury will put out the same damage as your Hunter's Marked d10 double Attacks and still give you an extra free attack, so it comes out ahead. ASIs: Mounted Combatant > STR*3 > WIS/another feat. Damage at 11: ~13+4 + ~13+4 + 1d10+3+2+1d6 = ~48 damage each round. Damage at 20: ~13+6 + ~13+6 + 1d10+1+5+3+2+1d6 = ~58 damage. Can go with Lucky instead of WIS to make up for no Advantage opportunities. This build suffers because you have nothing good to do with your bonus actions. Not a terrible option, but there are other builds with comparable damage that leave you much less vulnerable.
(8) STR dual Longswords with Two-Weapon Fighting, Dual Wielder, and a Giant Poisonous Snake. ASIs: Dual Wielder > Mounted Combatant > STR*3. The problem here is the same as before: Two-Weapon Fighting requires you to take the Attack action, so this doesn't synergize with Bestial Fury. It's either ~13+4 + ~13+4 = ~34 from a snake Bestial Fury or 1d8+2+6 + 1d8+2+6 = ~25. There's literally no point to taking a second Longsword over a Shield or going Versatile, because you're not out-damaging a Bestial Fury.
(9a) STR Quarterstaff and Shield with Dueling Style, Polearm Master, Mounted Combatant, and a Giant Crab. Okay...take a deep breath. Here we go. I considered making use of Versatile and dropping the Shield altogether, but the Dueling bonus only kicks in if you use the staff 1-handed. We could go with Defense fighting style instead of Dueling to make use of Versatile, but if you do the math: 2 AC from a Shield beats out 1 AC from the style, and 2 damage from Dueling beats out 1 average damage from the d8 vs d6. So keep the Shield and stick with Dueling style. Using a Shield means we can take Shield Master, but it's not ideal because we have competition for our bonus actions now: either Polearm Master's bonus attack or Shield Master's Shield Bash. So I dropped Shield Master. The original goal was to make use of Sentinel+Polearm Master to lock foes down at 10ft range for Bestial Fury (Giant Poisonous Snakes have 10ft range). Unfortunately, Quarterstaff only has 5ft range (despite being mentioned in Polearm Master). Also, when you Bestial Fury, you can't get the Polearm Master extra attack, so I had to make a choice: do more damage with the snake or focus on tanking and drop Bestial Fury. This build isn't going to out-damage the dedicated damage builds, so I chose to stop trying to max my damage and start to max my utility. Plus, Sentinel won't let you Bestial Fury for Opportunity Attacks, so we gotta get away from relying on our pet for damage. I dropped Sentinel because Quarterstaff means they'll be in 5ft range anyway... And I also dropped the snake and picked up a crab because it's tanky and can Grapple. I would still Bestial Fury now and then with the crab to Grapple some foes, but I'm not going to build around that. That's kind of just a bonus to this build. Instead, we rely on the extra Polearm Master attack that's boosted by Dueling style and Hunter's Mark. ASIs: Polearm Master > Mounted Combatant > STR*3. You can take Shield Master in place of a STR ASI if you really want extra tankiness, but I don't think it's needed. Hell, the crab's tanky enough that you might not even need Mounted Combatant! The only thing I'd be worried about are AOEs, which both feats help to mitigate. I'd still lean towards Mounted Combatant rather than Shield Master, just so you don't have the competing bonus actions as I mentioned above. The most beautiful thing about this build is that it reaches its mechanical potential once you get Extra Attack at level 5. Yes, level 5!!! Damage at 5: 1d6+2+2+1d6 + 1d6+2+2+1d6 + 1d4+2+2+1d6 = ~33 damage. Damage at 20: 1d6+1+5+2+2+1d6 + 1d6+1+5+2+1d6 + 1d4+1+5+2+1d6 = ~47 damage. Ehhh... The damage just isn't there. To be fair, you'll be getting a lot of extra damage from OAs, but you'd have to get an OA on every single round to catch up in damage to the other tank builds. At 5: ~33 + 1d6+2+2+1d6 = ~44 damage. At 20: ~47 + 1d6+1+5+2+1d6 = ~60 damage. Let's say you can OA 50% of the time and take an average. Average at 5: ~39 damage. Average at 20: ~54 damage. If you're confident you can pick up a bunch of OAs, then I guess this is an acceptable tanking option. It actually out-damages a few other builds by level 20! It all depends on how many OAs you can get.
EDIT: added build (9b)
(9b) STR Lance and Quarterstaff with Defense style, Sentinel, Polearm Master, and a Giant Crab. The problem with the build above (9a) was that Quarterstaff didn't have 10ft reach, so we couldn't make good use of Sentinel. Well, lucky for us, we can 1-hand both a Lance (while mounted, which we are) and a Quarterstaff. Now we can do the infamous Sentinel+Polearm lock-enemies-down-at-10ft OA combo because we have a Quarterstaff equipped and they enter our range at 10ft due to our Lance. It'd be so much easier if we could just use a Heavy polearm, like a Glaive or something, but nope. We're Small sized and we have to find crappy loopholes like this. Anyway, by locking enemies down 10ft away with our Lance, we don't have to worry about taking Mounted Combatant or Shield Master to protect our pet: only ranged attacks and AOEs will reach, and even when they do, it's a crab with high AC, so it's durable. By the way, we have to take Defense style, because Two-Weapon Fighting isn't useful for this build and Dueling requires only 1 weapon... Another downer is that we have to wait until 8 for the 2nd feat. Still faster than the Bestial Fury builds I suppose. ASIs: Polearm Master > Sentinel > STR*3. Half the time, you'll be doing damage with Lance at 10ft range and half the time they'll get through and you'll be doing damage with Quarterstaff+Polearm Master bonus action attack, so we'll count both and take an average. Damage at 8 (Lance): 1d12+2+1d6 + 1d12+2+1d6 = ~24 damage. Damage at 8 (Quarterstaff): 1d6+2+1d6 + 1d6+2+1d6 + 1d4+2+1d6 = ~26 damage. Average at 8: ~25 damage. Eww... Jebus, this build is awful. The Quarterstaff even out-damages the Lance! Yeah, you can finally do the Sentinel+Polearm Master+10ft range combo, but at what cost?! If you really wanna build a tank around this combo, don't pick a Small sized race.
(10) DEX Rapier and Shield with Dueling style and a Giant Crab. Anytime you don't see Giant Poisonous Snake, it's because we're trying to tank, not maximize damage. ASIs: Shield Master > Mounted Combatant > DEX*2 > WIS/another feat. Use first round bonus action to Hunter's Mark, then Shield Bash each round afterward to keep enemies Prone for constant Advantage. As we learned earlier, Bestial Fury will out-damage your Extra Attack, even with crappy pets. So just Shield Bash and follow with a Bestial Fury each round. Your damage isn't going to be great, but you're controlling the entire battlefield and tanking like a champ. Damage at level 11: ~5+4 + ~5+4 + 1d8+3+2+1d6 = ~31 damage each round. Damage at 20: ~5+6 + ~5+6 + 1d8+1+5+3+2+1d6 = ~41 damage. Really low damage, even for a tank... There are a lot of Grapples and Prones for Advantage, but I don't know if that's enough to make this build worth it...
(11) DEX Rapier and Shield with Dueling style, Defensive Duelist, and a Giant Poisonous Snake to make up for our lack of damage. Giant Poisonous Snake's Bestial Fury is powerful, combined with occasional Shield Bashing for Prone and Advantage, makes this build very strong. ASIs: Shield Master > Mounted Combatant > Defensive Duelist > DEX*2. Damage at 11: ~13+4 + ~13+4 + 1d8+3+2+1d6 = ~47 damage. Damage at 20: ~13+6 + ~13+6 + 1d8+1+5+2+2+1d6 = ~56 damage. This is really nice damage coming from a tank build! You should consider dropping Dueling Style for Defense style, since the snake is doing all the damn damage anyway.
(12) DEX dual Rapiers build with Two-Weapon Fighting style, Dual Wielder, and a Giant Poisonous Snake. Again, Two-Weapon Fighting doesn't stack with Bestial Fury, so this is out.
(13) DEX dual Rapiers build with Two-Weapon Fighting style, Dual Wielder, Defensive Duelist, and a Giant Poisonous Snake. Same as above (build 12). Nope.
(14) Ranged DEX Crossbow build with Archery style, Crossbow Expert and a Pteranodon. Bestial Fury and the bonus action attack from Crossbow Expert don't work together, so we're going for 3 Hand Crossbow attacks here: Attack, Extra Attack, bonus action Attack, all with Hunter's Mark. But wait, hold on, let's make sure it's worth giving up snake Bestial Fury before moving on with any of the ranged builds. So again, we want to compare the 2 snake bites you'd get from Bestial Fury to 2 crossbow attacks (Extra attack + bonus action attack) and see which comes out ahead. Hand Crossbow damage at 11: 1d6+3+6 + 1d6+3+6 = ~25 damage. Snake damage at 11: ~13+4 + ~13+4 = ~34 damage. 25 vs 34. Bestial Fury wins! But wait...that's only a 9 damage difference, which is less than a single Sharpshooter bonus (+10). So it looks like, if we can find a way to use at least 1 Sharpshooter effectively, then finally, something can beat out Bestial Fury! Archery style is giving us +2 already, so that makes the -5 from Sharpshooter 40% less bad. So, we can effectively Sharpshooter 40% more and still come out even. So let's do it. Technically, we're only using it 33% more when we add Sharpshooter to one of our three attacks, so the math is still in our favor. But wait, there's more! You can still Sharpshooter the free ranged attack you get from Bestial Fury! And if we're still doing it 33% of the time, that's another effective 3.3 damage added to Team Bestial Fury! So Bestial Fury barely pulls ahead again 35 vs 37! The reality is, the damage is so close either way... So is there a reason to lean one way or the other? Well, Bestial Fury requires you to put yourself in melee range. Without it, you can stand back and take less damage, so that's a perk. Plus, if you can somehow get Advantage, then you can put Sharpshooter on all 3 attacks instead of just 1 and actually pull far ahead of Bestial Fury in damage. That's another perk. So let's go with Sharpshooter instead of Bestial Fury. As a note, if enemies manage to get into 5ft range, you don't have to worry about Disadvantage because of Crossbow Expert. Since we're not relying on Bestial Fury at all in this build, we can drop the snake and pick up something more advantageous for ranged users. Try for a Pteranodon, otherwise a Giant Wolf Spider will do. ASIs: Crossbow Expert > Sharpshooter > Mounted Combatant > DEX*2. Damage at 8: 1d6+3+1d6+10 + 1d6+3+1d6 + 1d6+3+1d6 = ~40 damage. Damage at 20: 1d6+1+5+2+1d6+10 + 1d6+1+5+1d6 + 1d6+1+5+1d6 = ~51 damage. That's actually kinda low, so let's change a couple things for optimization. Let's go with Swift Quiver instead of Hunter's Mark. Sharpshooter 40% of the time averages +4 to each attack. Finally, let's switch to a Light Crossbow, since we're not using the bonus action attack once we gain access to Swift Quiver. Now, damage at 20: 1d8+1+5+2+4 + 1d8+1+5+4 + 1d8+1+5+4 + 1d8+1+5+4 = ~60 damage. This is an acceptable build, but not as good as the melee options. You reach your potential at 8 instead of 11 because you're not waiting for Bestial Fury, so there's that. If you can find a way to get Advantage (Faerie Fire, Inspiration, surprise, etc), then you can throw in a few more Sharpshots to boost this damage significantly. Sharpshooter on all 4 shots would bring the average damage at 20 up to ~84! Ranged builds are just super reliant on Sharpshooter to compete with Melee builds.
(15) Ranged DEX Crossbow build with Archery style, Crossbow Expert, Lucky and a Pteranodon. Variant to above but drop Mounted Combatant for Lucky. Giant Wolf Spider if you can't get a Pteranodon. The main reason we're taking Lucky is to make our Sharpshots more reliable, but it should be noted that it will also help us when we really need to land a skill check or save. Mounted Combatant isn't a priority because you're actively avoiding melee range anyway... You could, theoretically, take both Lucky AND Mounted Combatant, and drop a DEX ASI. But since we're relying so much on our own damage and need to be more accurate to land our Sharpshots, let's just go with 2 DEX > Mounted Combatant (I can't tell you how often I was just 1 point off from an enemy's AC). Without Mounted Combatant, our pet still boosts our speed and potentially give us flight or wall-climbing, so we still benefit from being Small and riding it. Worst case scenario though, we'll still be just as effective with this build if our pet dies. Sharpshooter lets us attack from long range without Disadvantage, so just run to 120ft with a Hand crossbow or 320ft with a Light Crossbow and drop Mounted Combatant altogether. ASIs: Crossbow Expert > Sharpshooter > Lucky > DEX*2. Damage at 8: 1d6+3+1d6+10 + 1d6+3+1d6 + 1d6+3+1d6 = ~40 damage. Same swaps as above build (14) at level 20: Light Crossbow, Swift Quiver, and 40% Sharpshot instead of 33%. Damage at 20: 1d8+1+5+2+4 + 1d8+1+5+4 + 1d8+1+5+4 + 1d8+1+5+4 = ~60 damage. Literally, the same damage as above (build 14), but more reliable because of Lucky. No biggie if your pet dies, but stay the hell back anyway. The point of going Small is to ride it in the first place, so at least try to keep it alive. After all, if you're not planning to ride your pet, why are you limiting yourself to Small races? And like above, the more Sharpshots you can squeeze in, the better, so looks for other (non-Lucky) ways to get Advantage as well. Firing from long range (yay Sharpshooter) can often give you Advantage on a surprise round. Again, 4 Sharpshots would bring average damage at 20 all the way up to ~84 each round.
(16) Ranged DEX Crossbow build with Archery style, Crossbow Expert, Sharpshooter, Lucky, and a Giant Poisonous Snake. Let's see what the damage would be with snake Bestial Furies. Start with a Light Crossbow from levels 1-3 for the larger d8 damage. At level 3, your snake will out-damage you, so just command it to attack instead of you. At level 4, you pick up Crossbow Expert and should switch to a Hand Crossbow so you can bonus action attack all the way to 10. Alternatively, you can command your pet to Attack and then follow with a free attack, but then you lose your bonus action attack. I'd stick to Extra Attack during these levels. Light Crossbow from 11-20 again because you're going to Bestial Fury and can't bonus action attack anyway. Swift quiver at 17 gives this a big boost, especially with a Light Crossbow instead of Hand Crossbow. You still need Crossbow Expert to remove Loading for Swift Quiver to work, but we've had that feat since level 4. You can Bestial Fury from 10ft range (snake has 10ft reach), but even if you have to get into 5ft range, Crossbow Expert removes Disadvantage, so just ride the snake the entire time. Unfortunately, you can't do anything with your bonus actions from 11-16, but that's okay, because Bestial Fury makes up for the damage. With Swift Quiver up, you get off a snake Bestial Fury AND 3 Light Crossbow attacks! Like I showed in build (15), the math is in your favor to use Sharpshooter on that free Bestial Fury shot 40% of the time. In fact, I'm gonna add +4 (40% of Sharpshooter's +10) when calculating damage at level 11, and add +10 (for one use of Sharpshooter per round) when you Swift Quiver 17+. You could drop a DEX ASI for Lucky. I wouldn't drop Mounted Combatant though since your pet needs to get close to enemies for Bestial Fury, and you can ride it because Crossbow Expert lets you shoot from 5ft. Most of the damage from the snake at this point too, so dropping 1 DEX for ensuring you'll land a few Sharpshots is worth it. Let's go with Lucky right after Sharpshooter, and take Mounted Combatant early so you can sit comfortably in melee range and keep your pet safe. ASIs: Crossbow Expert > Mounted Combatant > Sharpshooter > Lucky > DEX. Damage at 12 (gotta wait til 12 for Sharpshooter): ~13+4 + ~13+4 + 1d8+3+1d6+4 = ~49 damage. Damage at 20: ~13+6 + ~13+6 + 1d8+1+4+2+10 + 1d8+1+4 + 1d8+1+4 = ~79 damage. Whoa. It's too bad that you can't Bestial Fury and still get your bonus action Hand Crossbow attack in levels 11-16 or the damage would be even higher for those midrange levels. Also, if you're relying on Bestial Fury, your pet has to be in close range, and you can't protect it with Mounted Combatant until level 8... That leaves your pet vulnerable for 5 levels of the game (you're not gonna be putting out these high numbers if your pet keeps dying, right?). If you're starting high level, this is the best build out there, but if you're not, just be mindful of the fact that your pet will be somewhat vulnerable for a few levels. Getting Sharpshooter on all 3 bolts when you get Advantage will boost damage by another 12, for ~91 damage a round!
(17) Ranged DEX Light Crossbow with Crossbow Expert, Sharpshooter, Lucky, and a Pteranodon. This build focuses on using the bonus action pet Help to give you Advantage so you can Sharpshooter more often. Flyby Help with a Pteranodon, or if DM won't allow it, then send a Giant Crab in alone. You could ride a crab into 5ft range because of Crossbow Expert negating Disadvantage, but this build is very feat-heavy as is, so we don't get access to Mounted Combatant until very late. Might as well just not ride the pet so we don't have to take Mounted Combatant at all (we really need the DEX since we're using Sharpshooter more often than the other builds). Sharpshooter 100% of the time on the first attack (because of Advantage) and 40% of the time on the Extra Attack (so +4 when doing math). It's a more reliable build than the ones above, but you're going to want to Swift Quiver at 17+, so no more Advantage/reliability. You also lose the Hunter's Mark boost at 20. ASIs: Crossbow Expert > Sharpshooter > Lucky > DEX*2. Damage at 8: 1d8+3+1d6+10 + 1d8+3+1d6+4 = ~36 damage. Damage at 20: 1d8+1+5+2+4 + 1d8+1+5+4 + 1d8+1+5+4 + 1d8+1+5+4 = ~56 damage. By 20, this catches up to the the other builds, but this build falls behind too much for most of the game.
(18) Ranged DEX with Shortbow and Archery style, Sharpshooter, Lucky, and a Pteranodon. The good news about using a bow is that we can drop the Crossbow Expert feat. Unfortunately, we don't have access to the Heavy, better Longbow. We also lose out on Crossbow Expert's bonus action attack, so if this build is gonna be good, then Sharpshooter needs to land. This build relies on the Pet Help bonus action to give us Advantage enough to proc Sharpshooter often. The Lucky feat will help us out a lot too. Unfortunately, our pet has to get into 5ft range for the Help action, so Pteranodon can flyby or a Giant Crab can go in safely alone (don't ride it). This build doesn't require a lot of feats, so we can cap DEX faster. Mounted Combatant isn't necessary because we're not riding our pet if it isn't a Pteranodon, and even then, you can just fly out of range of danger most of the time. Sharpshooter 100% of the time on the first attack (because of Advantage) and 40% of the time on the Extra Attack (so +4 when doing math). Swift Quiver as your bonus action will out-perform Advantage on the first attack, so 17+ we stop making our pet use the Help action. This build is not spectacular, but it caps mechanically really early, at level 7, or even level 5 if you don't want to Sharpshooter 100% of the time on the first attack. ASIs: Sharpshooter > Lucky > DEX*2 > WIS/something else. Damage at 7: 1d6+3+1d6+10 + 1d6+3+1d6+4 = ~34 damage. Eww. Damage at 20: 1d6+1+5+3+4 + 1d6+1+5+4 + 1d6+1+5+4 + 1d6+1+5+4 = ~57 damage. Meh. Okay by 20, but might as well go Crossbow.
(19) Ranged DEX with Shortbow, Sharpshooter, Lucky, Crossbow Expert, and a Giant Poisonous Snake. Instead of relying on Extra Attack with build (18) above, we rely on Bestial Fury for bigger damage. The problem again though, is that you don't have Crossbow Expert for negating Disadvantage at 5ft range. So you can't ride your snake and protect it with Mounted Combatant. I mean, you could start out at 10ft range and Bestial Fury because the snake has reach, but then when enemies run into 5ft range, you're boned. Yes, your snake will do a lot of damage on its own (way more than you with a crappy Shortbow), but having Disadvantage half the time is bad. Might as well go dual Lances for the higher hit dice (build 4) if you don't mind the Disadvantage. There are two options: 1) take Crossbow Expert literally for the 5ft range aspect so you can ride your snake and keep it alive, or 2) send in your snake alone with no way to protect it. Remember folks: dead pets do ~0 average damage. So let's take Crossbow Expert (ugh) and see what numbers we can pump out. And for the record, your snake is gonna out-damage you early on, so at level 5 when you get Extra Attack, you should command it to attack and get a free bow shot in afterward. And if we're doing that, then the snake has to be in melee range early on, so might as well take Crossbow Expert early to negate that occasional Disadvantage (double ugh). You're gonna need Mounted Combatant early too (triple ugh)... ASIs: Crossbow Expert (yes, take it first...) > Mounted Combatant > Sharpshooter > Lucky > DEX. Yep, you can't cap your DEX anymore. You could drop Lucky for DEX, but I wouldn't recommend it. Better you land Sharpshooter 40% of the time and rely on your pet for most of the attacks. Damage at 12 (gotta wait for 12 to get Sharpshooter): ~13+4 + ~13+4 + 1d6+3+1d6+4 = ~48 damage. Damage at 20 (with Swift Quiver 33%): ~13+6 + ~13+6 + 1d6+1+4+2+10 + 1d6+1+4 + 1d6+1+4 = ~76 damage. What the hell? Damn, snake, you scary. Crossbow still comes out a whopping 1-3 damage ahead, but this is a surprisingly good alternative.
(20) DEX Hybrid Rapier and Hand Crossbow with Two-Weapon Fighting style, Crossbow Expert, and a Giant Poisonous Snake. Best of both worlds and versatility on the battlefield. Crossbow Expert gives you that extra bonus action attack, even if your main hand weapon isn't Light. Make sure your DM is cool with RAW so you don't need a free hand to reload the Hand Crossbow (Crossbow Expert RAW removes Loading property). This build is still held back by the fact that Bestial Fury doesn't synergize with Crossbow Expert (requires Attack action to get the bonus action attack). Two Rapier swings ain't gonna out-damage your snake. Still, having the option to melee or ranged when needed is nice. Without that extra bonus action attack, I can already tell you that it's not going to out-damage the Light Crossbow build (14), and frankly, Crossbow Expert's 5ft range feature makes this hybrid concept pointless if you can't get a better mainhand weapon. Not even gonna do the math.
(21) STR/DEX Hybrid Lance and Hand Crossbow with Two-Weapon Fighting style, Crossbow Expert, and a Giant Poisonous Snake. Let's try this with a stronger mainhand weapon. Kinda MAD. Bestial Fury doesn't synergize with Crossbow Expert, so the question becomes: do we have to rely on Bestial Fury? Do 2 snake bites out-damage 1 Lance attack + 1 bonus action Hand Crossbow attack? ASIs: Crossbow Expert > Mounted Combatant (tempted to drop this but it's just so good) > DEX > STR > DEX. You'll be using DEX more than STR because you'll usually be stuck in 5ft range. At level 11, 2 snake bites: ~13+4 + ~13+4 = 34 damage. At level 11, 1 Lance attack + 1 bonus action Hand Crossbow with Hunter's mark does: 1d12+2+6 + 1d6+3+6 = ~27 damage. The snake wins! By 20, you catch up a bit: 1d12+1+3+2+6 + 1d6+1+5+6 = ~34 damage. Oh wait, your snake get another +2*2 from your higher proficiency bonus by 20, so actually the snake still wins. Plus, like I said earlier, you'll probably be in 5ft range a bunch, so your damage goes down even more if you can't Lance.
IX. Conclusion
So what did we learn? Giant Poisonous Snakes put out a lot of damage, so Bestial Fury builds tend to shine. Unfortunately, a lot of our options, such as Two-Weapon Fighting's bonus attack and Crossbow Expert's bonus attack, don't synergize with Bestial Fury.
The early feat requirements of the STR builds leaves you with a 14 STR for 11 levels, making the Bestial Fury all the more important. Your Small size also severely limits your weapon choices. If you're going STR, Lance and Shield (build 2) is the best Lance build, but missing a Shield Bash shove will punish you with Disadvantage. I would just avoid Lances altogether. Dual Lances doesn't work with Bestial Fury, so no help there. The Longsword builds don't let you take Defensive Duelist, so even when using Versatile, you are taking too much damage to be optimal. Shortsword and Shield (6) will be more reliable and lets you tank well. The Quarterstaff Sentinel build (9a) is actually a great tanking option as well, but relies heavily on Opportunity Attacks to make its damage acceptable.
If you're going DEX, congratulations, you have more wiggle room. The racial DEX boost opens up extra feats and maximizes damage faster. Dual Rapiers is out, but the Rapier and Shield with Defensive Duelist (11) puts out great numbers while still being tanky. It ends up being slightly more optimal than build (6) above.
Your Small size prevents you from using Heavy Crossbows and Longbows, so the ranged options suffer. The Crossbow Expert feat's wording prevents synergy with Bestial Fury, so that messes things up a bit too. Sharpshooter is amazing though, and can keep up with Bestial Fury damage if you throw it on more than 1 attack per round. The bonus action pet Help builds are meh. The best numbers come from using Crossbows (alternate between Light Crossbow and Hand Crossbow) and relying on snake Bestial Fury for damage (16). Shortbow (19) can almost keep up, so don't dismiss it, but if we're going for optimal then stick to Crossbow.
Hybrid melee+ranged builds just suck.
tl;dr
DEX>STR. Ranged>Melee. Bestial Fury>Extra attack. 2+ Sharpshooter>Bestial Fury>1 Sharpshooter. Lance is a trap (gotta worry about Disadvantage too much). Build (9a) is best for straight tank (great control), build (11) is best for general melee (balances good damage with tankability), and build (16) is the best ranged (balances high damage with survivability). Overall, build (16) puts out the biggest numbers without succumbing to glass cannon-ism, so that's technically the most optimal.
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