Cerberus

Cerberus is Hades's pet three-headed dog who can be found exclusively as a boss in the Kingdom Hearts series. It appears in Kingdom Hearts, Kingdom Hearts II, Kingdom Hearts coded, and their remakes.

Kingdom Hearts
''Guardian of the entrance to the Underworld and Hades's loyal servant. Herc defeated him once, but Hades brought him back to fight Sora and his friends.

He first stood watch Hades in "Hercules" (1997).''

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 one of four sources of the Bug Blox corruption in the Olympus Coliseum.

Appearance
Cerberus is a massive, three-headed dog with black fur and short, cropped ears. His heads are identical to each other, each sporting the same enormous mouths, pink tongues, and long, sharp teeth. All three heads also feature glowing red eyes that slant downwards, imposing as a menace. Cerberus's legs are rather short, and each ends in a paw with three grey claws. He also has a thin tail and seems to be quite muscular.

Personality
Cerberus has an angry and feisty personality, but will obey his master, Hades, whenever he is called into battle, as portrayed in Kingdom Hearts and Kingdom Hearts II. However, he will retaliate if Hades beats him too much.

Origin
In Greek mythology, Cerberus is a three-headed dog, child of Echidna, sibling to the Hydra and several other mythical creatures, such as the Sphinx and Chimaera, that guarded the gates of the Underworld and prevented anyone from escaping past the River Styx. As Hercules's twelfth labor, he was to capture Cerberus and bring him back to King Eurystheus. Hercules came down to the Underworld and forced Hades to allow him to take Cerberus, who was too afraid of Hercules to resist. When Hercules showed the King, he hid in a pot and told Hercules to put Cerberus back in the Underworld.

In the animated film, Cerberus plays a rather minor role, guarding the River Styx. Near the end of the movie, Hercules rides on Cerberus to Hades's palace.