Well, we know animal transformations are possible. Moody/Crouch turned Draco into a ferret. So it's conceivable there is a potion that can do the same thing. It's probably something similar to Polyjuice Potion, but I doubt it's simply making Polyjuice Potion with animal hair instead of human hair.
Originally Posted by OliveOil_Med
I also suspect it's a lot more dangerous, kind of like becoming an Animorphmagus. See my earlier post; if you drink a potion that suddenly turns you into a cat or a ferret or a bird, you're going to be in a body that you have no idea how to control. And it's possible that becoming an animal could have other side effects that make the whole thing more dangerous.