
Here are your choices for familiar (as of Alpha) Toad, Weasal, Cat, Raven, Hawk, Owl, Fish, Bat.
If your a Chainlock you also have Imp, Quasit, Pixie, Puesoduedragon.
As an action you change the shape of your familiar into that of another familiar you have access too (so a regular familiar could take the shape of another animal familiar, but a Chainlock familiar can take those shapes or the shape of the special familiars it has access too) for an hour, it takes a short rest before you can do that again. You can use your familiars senses if its within a hundred feet of you. You have telepathy with it. It can deliver touch spells. Chainlocks can get an invocation, voice oc the chainmaster, to do more like giving one of your attacks to your familiar, speaking through it with your own voice and more. Familiar's normally can't attack, but I believe they do get an action which means it can do stuff like dodge, dash, and hide, ect... oh and the are fey, celestial, or fiends, instead of beasts, not sure if that's just a keyword for beings with those types or if its a template. The alpha doesn't have the stats for these, as it doesn't have the creature appendix.
Oh and if your familiar dies it can be summon back after a short rest with a minor ritual.
Steeds, regular steeds you buy, instead of summon, do not have attacks.
The Find Steed Spell grants you a war horse, stag, pony, war dog. Its regulur origin (usually beast) is replaced with fey, celestial, or fiend type. When used in a 4th level slot it binds a Giant Bat or Giant Eagle to you. If you cast it with a 5th level slot, you can summon a nightmare or unicorn. We don't have the stats for these creature.
You magic steed has a minium of 6 intelligence, if that boosts the intelligence of that animal type to sentience it gains a language you know. You and your steed have telepathy out to a mile. Any time you cast a spell that effects only you, your steed can benifit too. So casting divine power on yourself means it benifits your mount too. Same with casting cure wounds on yourself, you heal yourself, but you heal your mount too. Spells like smites have range of self, so I'm not sure, but your mount might be able to when you do, I'm not certain.
As others have mentioned the mounted combat feat its awesome for protecting your mount, but it gives advantage against enemies smaller then your mount as well.
Also once a mount is bound to you, if you dismiss it or it dies you can resummon that same mount with the find steed spell. This is extra cool, because if you say summon a unicorn with a 4th level slot, to summon it again later, you use a 2nd level slot, because once you have a mount bound to you, find steed summons the steed bound to you, not a new one.
Find Steed is also a ritual, but that is almost useless because Paladin's don't get ritual casting and the ritual caster feat don't let you choose paladin rituals.
Your steed is almost always close to you, which means it benifits from your auras, like Aura of Protection.
Steed can also use barding, of the same types as humaniods, so you can give your steed plate barding for an 18 AC. Its for 4 times as expense and 4 times as heavy as the regular verison of armour/barding you buy. A military saddle gives you advanage if you get knocked off your mount.
If your steed pulls a wagon, cart or carriage ect...it can pull 4 times its normal carry capicity.
The really big thing is is that intelligent mounts get thier own turns and can do whatever it wants. With a none summoned intelligent mount this can be a double edged sword, the mount can say bite me and ignore your wishes, but with find steed, your mount is loyal and you fight as a single unit, so your mount disobeying orders is rare, but your mount can attack on it own enemies, none intelligent mounts can't attack.
Read the mounted combat rules in basic to see what I mean.