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 acts as a back-up plan for Hades, if Cloud, whom Hades promised to help find Sephiroth for him, fails his task of eliminating Hercules and Sora.
When Sora and Cloud's battle comes to an end (regardless of who wins), Hades sees that Cloud is no longer of any use to him, and sends Cerberus in. Cerberus manages to pin Cloud to the ground when his back is turned, and is about to attack the others, but is held off by Hercules, allowing Phil, Sora, Donald and Goofy to escape.
Cerberus has Hercules, who is carrying Cloud, pinned, but Sora comes to help, returning the favor and allowing Hercules to escape. Sora manages to defeat Cerberus, and his efforts reward him the status of "Junior Heroes". However, as Sora leaves, Hercules mentions that he had already worn Cerberus down by the time Sora intervened.
Cerberus can be fought again in the Hades Cup, except he has four health bars instead 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, but ends up separated from Donald and Goofy, leaving him and Auron to fight the boss by themselves.
The two then manage to get away, and Sora, Donald, and Goofy taunt Cerberus with faces, and the door separating the trio and Cerberus closes just as it attacks.
Cerberus has his own cup in the Underworld. The final round of the Cerberus Cup is a timed fight against Cerberus.