Players will be adjusting their looks, maybe you could just add an option down at the bottom to have the animal effect added as you log in?

"If you were playing as an animal before, (or want to take a walk on the wild side) then select one of the options below to start the game as an animal. "
"Warning: This change may be difficult to undo. Only choose the option to start the game as an animal if you fully understand the implications. Changing back from an animal form may involve killing a boss, or finding a rare or expensive potion".

Existing players should know the implications.

A new player has little risk, they can just make a new character.