So, last week I talked a little about exotic characters, mostly of the centaur variety. Those are tricky, but mostly humanoid, at least from the waist up.

So, the logical next step is to point out that occasionally, players will get it into their heads they want to play an animal. Most will probably go easy on you, and want to play something Awakened, or in a campaign where literally everyone is playing an animal of some kind (Ponyfinder, Tails of Equestria, or one of their cousins), but every so often, somebody will get this idea for a character for a campaign that isn’t that easy to fit an animal into.
I’ve seen people play squirrels. I’ve seen people play dogs. I’ve played a dragon. Any of these has tricky bits, for the GM, and for the player. For the GM, you will want to remember that most people see an animal and … it’s an animal. If it’s a dog, and it isn’t collared, it’s probably a stray, even if it is well-behaved. If it’s a squirrel, it’s cute at best, and a menace to the bird-feeders at worst. No one is automatically going to recognise it for the special case that it is, even if it is something like a Blink Dog, Winter Wolf, Unicorn or a talking fae-touched squirrel rogue.
So, because that does sound a bit judgmental, I am going to come right out and say it. If you have a player who wants to do that? There’s nothing wrong with that. We push boundaries, we explore, we poke things. There’s nothing wrong with that. If it doesn’t work for your campaign, say so, because there’s nothing wrong with that, either.
However, we’re assuming there is. So, you have… let’s go with the squirrel, because he’s, by most game rules that I am familiar with, tiny, or diminutive. Finding any gear that will fit him that isn’t custom made is probably not going to happen. He may be able to wear rings, but not as designed (and someone is going to notice that). He may be able to use some tools, but not as designed, and where’s he going to carry them? So, you’re going to have to do some serious thinking about how to handle that, because most systems are very heavily gear-based. Now, Pathfinder does have a ruleset that sidesteps that, but I don’t know 5e, and 3.5e does not have such a ruleset that I am aware of. 2e might. Other systems may not be as gear-dependent, but they may also not be as friendly to talking squirrels, or may be more friendly, depending on the setting you are using (alien races that look like squirrels will have gear sized for them). Or you’re using a system that assumes nobody has thumbs, and has been set up accordingly.
Something to keep in mind, for all those otherwise really cool characters that might never see the light of day. Above all? Have some fun with it!