Lunar Wilds: A Moon Druid Animal Guide
Additional commentary from Yunru on CR 0 animals, from a guide he started but never completes which he called Lunar Wilds: A Moon Druid Animal Guide:
Baboon - The Baboon has Pack Tactics, putting it above the deer in terms of combat, even if it's attack is weaker and hits less often (without advantage). You've a climb speed which could be very useful, but your HP is very low so try to avoid getting hit.
Badger - Now here's an interesting one. Basically useless in combat, but it has Darkvision, advantage on smell Perception checks and can Burrow. There could be a niche case in combat where you throw up a damage-over-time concentration spell and then burrow away. You're also Tiny, which helps you go places you otherwise couldn't.
Bat - Blindsight, Flight and advantage on hearing Perception checks. Great for outside of combat, useless in it (more so than the Badger, as less things burrow than do fly). You're also Tiny, which helps you go places you otherwise couldn't.
Cat - Another combat-useless creature, however the cat has quite the potential outside of combat. Cats are common enough in populated areas. Couple that will a Climb speed, decent stealth and perception and advantage on smell Perception checks and you've got yourself quite the stalker form. You're also Tiny, which helps you go places you otherwise couldn't.
Crab - Blindsight, a Swim speed, Amphibious and Tiny. Other than that there's nothing to be had here.
Deer - Deer are useless. They have high Dex and an okay Wis, but they have no special traits and terrible combat use. You're going to stick out like a sore thumb in populated areas and probably get hunted in rural/wild areas.
Eagle - Remember what I said about the Cat being a good stalker form? As long as you don't get shot down by someone, the same holds true for the Eagle. You're not Tiny, but you've a Fly speed, a high perception and advantage on sight Perception checks. You still don't have much use in combat though.
Frog - Darkvision, Tiny, Amphibious and a Swim speed. The Crab's better in basically every way, and that says something.
Giant Fire Beetle - Don't let the name fool you, you're Small. You have Blindsight and you can't help but give away your position. Avoid.
Goat - A certain simulator has proven how powerful goats can be. Unfortunately those breeds of goat aren't native to DnD. You can charge for okay damage and prone, and have advantage on Strength and Dexterity checks that would knock you prone, but that's the limit of your combat potential. You're HP's also not good, but at least you're a some-what common sight.
Hawk - See Eagle, only with a less damaging attack.
Hyena - Your HP could match certain PCs, if they had to roll for their level 1 HP. You have Pack Tactics but lack the HP to really use it. You've okay Perception, but the Cat, Eagle and Hawk have you beat there. Also you're not likely to be welcome in civilized areas.
Jackal - See Hyena, although at least you get advantage on smell and hearing Perception checks.
Lizard - Tiny, Darkvision and a Climb speed. Unfortunately that's all that's going for you.
Octopus - Low HP, good stealth and interesting actions. You're definitely a glass cannon though. Also, unless your campaign is an aquatic one, you're going to stand out when you're noticed.
Owl - Decent Stealth, decent Perception, advantage on hearing and sight Perception checks. Tiny. FLYS. Doesn't provoke OAs when flying away from enemies. You're damage and HP is useless, but if you want an infiltrator you could do worse.
Quipper - Yeah no, you're useless and immobile out of water and not much better in it.
Rat - Dark Vision and Keen Smell. That's all you've got going for you. Avoid.
Raven - Tiny, a Fly speed and decent Perception. You don't get advantage, but you can mimic what you hear (potentially, ETV).
Scorpion - Avoid. Just, avoid. Unless you're coming across one in the wild, then you can just ignore.
Sea Horse - Sea above, except you can't even survive on land.
Spider - Tiny, Darkvision and a Climb speed. A poor man's Cat.
Vulture - See Hawk.
Weasel - A +5 to Stealth?! Decent Perception too and you get advantage on hearing and sight Perception checks. An okay scout.