Alrighty, here are your snakes, seals, squirrels, snails (actually a shelled gastropod but we're all friends here) and zebras, uh, I mean 'striped horses'. See, they all start with 'S'!
A brief overview, right here: They're all animals, and they all start with 'S'. If not, see Rule 1.
Seals
These cute little guys spawn in frozen biomes (ocean, sometimes rivers). They'll spend their time on land (ice) and water but prefer the water. Breedable with seagrass and kelp, but will avoid players.
Snails
Completely awesome. Found in swamps, the big ones are tameable and breedable with 'shrooms. You can heal them with seagrass, 'shrooms, wheat, vines, seeds, that sort of thing.
Snake
Yes, Alex is correct to stay back. Leave them alone and they'll leave you alone, I think that's fair warning. You can find them in most forests (not the snowy ones). They will avoid players but if you get too close they'll come for you.
Striped Horse
Find these in savanna biomes. They are rideable, without a saddle, without taming. Don't ask me why.
Squirrels
Cute little furry things. What can ya say! They can be found in forests (not cold or extreme hills forests). Tameable with cookies or melons, also breedable with the same. They avoid players so you'll have to be clever about taming them.
