[tbone]

Just last month, a fellow in the UK used a narwhal tusk to help fend off a knife attacker. (Useful things, those narwhal tusks, if you happen to have one nearby.)

But as for using an actual animal as a hand weapon: The only case that comes to my mind is a rather grisly one I read a few years back involving - no surprise - chimpanzees. (The scariest animals out there, IMO.) Skip the following paragraph if you prefer your forum posts free of unappetizing imagery.

So a male chimp in the wild was observed snatching an infant chimp. For lunch. The infant's mother gave chase. The male decided to bludgeon her with the weapon he had at hand: the baby. He drove her off with his "club" and then ate it.

GURPS/DFRPG offer stats for chimps, carnivorous/gladiator gorillas, and the like, but I don't think I've ever used apes as monsters in a game. Some monsters are just too disturbing.