Overall they look great and I think you've gone well beyond the level of unique details that would be necessary here. Sure, different sizes would be cool, but it's not worth making new equipment sprites for each size. Not if you can already fit humans, dwarves, elves, and goblins in one equipment size and make it look good, at least, but if you decide you need sizes for them anyways, then I reckon it's definitely worth utilizing that for these guys too.
Dealing with specifics:
The emphasis on distinct and clearly defined pinions makes the birdman wings feel more like demon wings or dragon wings; the proportions draw too much from fantasy and not enough from reality. Look at the size and angle in particular. Of course it's gonna be hard to make them look super accurate with the amount of space you've got no matter what, but having them come to such a sharp point at the elbow while pointing out rather than down is definitely not doing you any favors; bird wings just don't work that way. The raven wings are an exception to this, so the simplest fix would probably be to just recolor those wings for the others; I would shift them one pixel up since they look more like they're behind him than attached to him, but they're definitely preferable to the other wing design even before that.
I agree with Manveru that the shading on the oriole could use another pass.
The leech is way too long. Some of them can stretch out but in general they're stubby little critters, assuming the worm man is meant to represent annelids in general, they certainly shouldn't be longer than that.
It's certainly weird for a skink to be green... And for that matter, for an anole to be purple. Did you just swap their labels by accident maybe? Not that skinks are purple either, but I guess that it could look brown in some context, which plenty of skinks are. The shape of the one labeled anole looks more like an anole than the one labeled skink, but it also looks more like a skink than the one labeled skink so I can't really tell based on that.
Pond turtles are also not typically very green. I guess some species can be, sort of, but definitely not that green. The art is really solid and looks great on the desert tortoise, but I would consider another pass on the coloring.
I'm not 100% on board with that chameleon's face, he's giving me pepe the frog vibes, but they're weird looking animals at the best of times, and very diverse.
The ostrich head doesn't capture their wedge-like shape, that big flat bit right in front of the eyes isn't accurate and makes it look like a goose. Even though they have proportionally smaller beaks, the swan head would capture the feel of an ostrich's head better than the one you've got now. Or, better, recolor the emu head for it and make the neck one pixel longer if you've got the room.
I appreciate that it's not easy to make these into cute little humanoids, but the roach and beetle heads don't look anything like roach and beetle heads. In fact they look less like roach and beetle heads than a human head does.
I suspect you have limited space to work with for technical reasons, but the snail and slug would look a lot more like snail and slug if their antennae weren't bent, since they don't do that in real life. You could also give them the two little feelers down below, if that wouldn't complicate the layering.
The gila monster also looks unnecessarily weird. I think because the angle of the forehead, especially, but really I think that position is just too ambitious for the amount of pixels you've got available. There's also no need to give them those glowing white eyes; I know the eyes would be hard to make out otherwise but they are in real life too. Finally, you've given it a skink's tongue – gila monster tongues are forked, so it you remove that bottom left pixel (assuming you don't redraft it entirely), you'll have improved it a lot.
Those are the things that stood out to me when evaluating it as I would if I was making the sprites myself, though I know if I do that it'll be useless to you for legal reasons. The mammals categorically look great and most of the reptiles do too, and the spiders and ants are adorable and the bird faces are super cute too.