Cerberus

Cerberus is a mythical three-headed demon dog that first appeared in Disney's Hercules, as guard dog to Hell. Hades used his pet in both Kingdom Hearts and Kingdom Hearts II.

Kingdom Hearts
''Guardian of the entrance of the underworld and Hades's loyal servant. Herc defeated it once, but Hades brought it back to fight Sora and his friends. It first stood watch for Hades in "Hercules" (1997).

Kingdom Hearts II
Guardian of the entrance to the Underworld and Hades' loyal "pet."

Hercules, Sora, Donald, and Goofy teamed up to defeat it last time, but Hades brought it back - and it's as dangerous as ever.

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, trapped, 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 to Phil that he had already worn Cerberus down by the time Sora intervened.

Cerberus can be fought again in full power at the Hades Cup, having four health bars instead of two (thus proving the fact that Hercules had worn it down).

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 monster by themselves. The two then manage to get away, and Sora, Donald, and Goofy make faces at Cerberus as the doors close, keeping the monster from attacking them anymore. Cerberus has its own cup in the underworld, being the final opponent of the Cerberus Cup in a timed fight.

Trivia

 * While Cerberus in Kingdom Hearts just looks like a vicious three headed hound, in mythology it is described as being a lizard with a huge tail and smaller as Hercules was able to drag it from Hell in his trials.
 * In Kingdom Hearts II, the fight with Cerberus in the first trip is one the fights where King Mickey is able to be used if Sora fails.