Birds, fish, reptiles, amphibians, insects? Sure. But no mammals.
So I had to google it. Apparently, there is a sloth that moves around so slowly moss grows all over it and it doesn’t care. So it may appear green, but only in the sense that it wears it.
Best I’ve got is sloths. And they’re only green because algae grows on them. And I know it sounds like cheating because they aren’t intrinsically green but before you completely discount it there are animals that wouldn’t be the color they are in a different environment. For instance, flamingos are only pink because of the seafood they eat. If fed a different diet they can be almost white.
Yeah fair. I had painted glass fish in my aquarium at one point and discovered the “paint” came from feeding them dyed food and eventually faded away when I gave them normal food back at home. They are naturally transparent for the most part which, frankly, I thought was cooler. I did have a gourami that was legit green though, as far as I could tell.