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 made a few appearances in the series as a rambunctious puppy that Hades has difficulties in controlling.

Cerberus largest role in the series was in the episode "Hercules and the Visit From Zeus" while plotting another scheme to take over Mount Olympus, Hades is interrupted by a playful Cerberus who makes off with a model of Olympus while attacking Pain and Panic. Later, Hades is dragged across the Underworld while walking the pup, when Cerberus discovers the Fates all seeing eye much to the annoyance of the three hags. Hades retrieves the Fate's eye from his monstrous mutt when he gets a glimpse of a mortal. The Fates reveal that mortal was Zeus. Seeing a golden opportunity to rid himself of his brother once and for all, Hades sends Cerberus after Zeus.

Cerberus later appears in the episode "Hercules and the Underworld Takeover". Cerberus appears alongside the Fates and Hade's other underlings snarling at Hecate until she commanded him to sit during her hostile takeover of the Underworld.

Cerberus makes a cameo in the episode "Hercules and the Comedy of Arrows". Pain and Panic get their hands on the extra loathe arrows Icarus created when Cupid tried to dispose of them. They return to the Underworld with the arrows much to Hades fury for mistaking the loathe arrows for Cupid's love arrows. Hades throws one of the loathe arrows out of the window of his lair on to a sleeping Cerberus now, fueled by the power of the arrow attacks Pain and Panic.

Kingdom Hearts series
Cerberus appears several times throughout the Kingdom Hearts series, usually as a servant of Hades.

In Kingdom Hearts, he ruins the Games by battling Hercules, although Sora, Donald and Goofy step in and defeat him. This deed earns them the titles of "Junior Heroes". Cerberus appears again as the twentieth opponent in the Hades Cup.

In Kingdom Hearts II, Hades sends Cerberus after Sora, Donald, Goofy and Auron, who were escaping from the Underworld. The chase ended with the quartet managing to get away from the beast after a battle.

Cerberus also makes a minor appearance in Kingdom Hearts coded as a data version.

Trivia

 * Although most of the world bosses in Kingdom Hearts: Chain of Memories are the same as in Kingdom Hearts, Cerberus is replaced by Hades, who was an optional boss in Kingdom Hearts.
 * The Cerberus is mostly based on the Greek mythololgical monster of the same name, they are both monstrous three-headed dogs and their job is guarding the entrance of the Underworld.