An example from my last 3.5 character:
In short, the concept was a barbarian hunter who in his youth had slain a mythical white ape. He still wears its pelt and has since then been cursed with awesome, to remind him of the shame of killing this holy creature. He has gained the ability to turn into twisted ape-like creatures, with changing features including horns, tentacles and so forth. To help get the idea across, the writeup included a picture of the
Fiendish Girallon miniature.
(In mechanical terms, he was a wild shaping ranger/fighter/warshaper/nature's warrior/master of many forms, and the pelt was a darkleaf breastplate)
The DM seemed to be okay with it and fun was had, but a few sessions in he up and decided that no, he won't turn into an ape-like creature with fins or wings, but a crocodile and a dire hawk respectively. If for whatever reason you do feel the need to cramp somebody's style, there are better ways to communicate that than to just suddenly say "no" when a player describes his action, especially after the shtick has already been well established.
cheers