I do believe it is impossible to even change what animal you become when you switch forms. I know you can change the colors and textures, but that is only on your side of the screen, unless Aga were to do it so everyone could see it.The only changing of animals you can do is what animal accessories your veno has. Bunny, two types of foxes, a deer, a cat.
There's also a bat-like thing. Bat wing ears and a devil tail.