Cerberus

Cerberus is a mythical three-headed dog that first appeared in Disney's Hercules. He was Hades guardian and servant and he appears as a boss in both Kingdom Hearts and Kingdom Hearts II.

Hercules (1997)
In Hercules, Cerberus only appears in the Underworld. He guards the gates to the Underworld, and appears as Hades' pet. At the end of the film, Hercules breaks into Hades' throne room on the back of Cerberus. He is not seen again after that.

Hercules (TV)
Cerberus made a few appearances in the animated TV series Hercules that was based on the film. He appears obviously smaller than he was in the movie.

Kingdom Hearts
In Kingdom Hearts, Cerberus is summoned by Hades to eliminate Sora. He attacks during Sora's fight against Cloud in the tournament. He knocks Cloud out and goes for Sora. Hercules intervenes and saves Cloud. Sora runs nto the arena to fight off Cerberus in a boss battle and defeats him.

Cerberus can be fought again in the Hades Cup, except he has four health bars istead of two.

Kingdom Hearts II
In Kingdom Hearts II, as Sora and Auron are escaping from the Underworld, Hades sends Cerberus to stop them. Donald and Goofy escape as Auron fights off the creature. Sora returns to aid him, and the two fight the boss by themselves.

Cerberus has his own cup in the Underworld. The final round of the Cerberus Cup is a timed fight against Cerberus.