Cerberus

Cerberus is Hades' three-headed guard dog whose main duty is to guard the entrance to the Underworld. He made his first appearance in Hercules.

Hercules
Cerberus first appears when Hades is riding down the River Styx and is fed a massive steak. Later, Hercules tames the beast and uses him to get to Hades inner sanctum, though this is only a temporary turn driven by fear.

Hercules: the Animated Series
Cerberus appears as a rambunctious puppy Hades has difficulties in controlling.

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 quickly crushes Cloud beneath its front paw 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). However, since the first time you may not have obtained Cure, it might seem easier the second time you fight it.

Kingdom Hearts II
In Kingdom Hearts II, as Sora, Donald, Goofy, 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 Cerberus is trapped behind the doors as they close, but not before Sora, Donald, and Goofy taunt him by making faces. Cerberus has its own cup in the Underworld, being the final opponent of the Cerberus Cup in a timed fight.

Kingdom Hearts coded
Data Cerberus appears as a threat to Hercules, Cloud and Sora. He is the one of the sources of the Bug Block corruption in Olympus Coliseum.