The Giant Squid is an antagonist from 20,000 Leagues Under the Sea. It attacks the Nautilus near the end of the film. The scene with it has since become one of the most famous ones in a Disney film.
Obviously, don't nominate any human characters. Hybrids like Cheetah or The Lizard are okay, as are shapeshifters with one set animal form (eg Professor McGonagall.)
