Hydra

"Will you forget the head-slicing thing!"

- Everytime Hercules chops off a head more grow back. The Hydra is a large, multi-headed female monster appearing in Disney's 1997 feature film, Hercules. In the film, Hades summons The Hydra to destroy Hercules, though Hercules manages to defeat it, earning him fame and adoration from the people of Thebes.

The Hydra starts out with only one head, but any time one of its heads is removed, three more replace it.

Physical Description
The Hydra is a massive, reptilian creature with dark purple skin. Its body is rather bulky and it crawls using two four-fingered hands with white claws. It has a long, thin tail and each time one of its heads are cut off, three more grow to replace it. Its number of heads can apparently grow exponentially in Hercules, though in Kingdom Hearts II its maximum is seven. All of the Hydra’s head are identical to each other, each sporting the same long, serpentine neck, large mouth filled with white fangs, orange eyes with slit pupils, and spikes lining the top of their heads down the length of their necks. When one of the Hydra's heads is cut off, it spews green blood. In the International releases, however, this is desensitized to be purple smoke instead, though it remains green blood for the initial cutscene. The Hydra's name references the Lernaean Hydra, a serpent-like water monster of Greek mythology. It originally had nine heads that would replace each other in the same manner as the Hydra in Hercules, and was killed by Hercules as part of his Twelve Labours.

Hercules
The Hydra is the first obstacle sent by Hades to kill Hercules. It is trapped under a boulder until Hercules unknowingly releases her in rescuing two boys from under the same boulder who turn out to be none other than Pain and Panic (Hades' henchmen) in disguise. The two thank Hercules before running off to Hades and leaving the hero to feel a false sense of accomplishment, oblivious to what consequences would come of his actions.

The Hydra surprises Hercules by approaching him from the back, forcing the hero into combat until the monster lunges at him and knocks his sword back. Hercules defenestrate the beast by hurling a boulder into her mouth, but she only crushes it an laughs. When the monster next strikes he holds her mouth open and slams her head into the ground, giving him time to grab his sword. However the Hydra wraps its tongue around ankle, hurls him into the air, and swallows him. While in the monster's throat, Hercules decapitates her with his sword from the inside-out. The hero is again lured into a false sense of security before, as Hades expects to happen, the Hydra sprouts three more heads. She chases down Hercules  and causes the spectators in the stands to scatter. Pegasus then lifts the hero off the ground and Hercules fights back, to little avail. With each head he chops off, three more grow.

It is not until he is pressed against a rocky cliff that Hercules sees any chance of victory. He punches the cliff and creates a rock slide, finally killing the Hydra and putting him in a new light before the people of Thebes, who quickly start him on his road to fame and fortune (Zero to Hero).

Hercules (TV series)
The Hydra only made minor appearance in the series. In the episode "Hercules and the Parents Weekend" The Hydra appears as a baby during Echidna song "What's a Mother To Do?" with the rest of her monstrous children.

In Hercules and the Long Nightmare, Morpheus the god of sleep younger brother Phantasos really wanted his brother's job and centuries later is still jealous. In an effort to make Morpheus look bad, he spreads his own blanket of nightmares over the world, causing Hercules and Phil and the rest of Greece to have bad dreams.,Hercules bad dream was facing The Hydra Hercules confronts it rather than running away, and it turns into Phantasos who was force to come to reality.

Kingdom Hearts II
The Hydra is a boss at Olympus Coliseum. Sora must battle with the help of Megara, Phil and Pegasus to defeat it.

Hades sent the Hydra to destroy the Olympus Coliseum and kill Hercules. Hercules had to fight it in the Coliseum and believed he had knock it out. However, it was in fact playing dead and started to destroy the Coliseum. Sora then fought it with his friends and beheaded it until it could no longer grow heads. However, the aftermath of the battle was rather unfavorable—the Coliseum was destroyed and Hercules lost his hero's glow, making him later fall prey to the plans of Hades.

The reason Sora didn't have the same trouble Herc did in the film may have to do with him slicing off its heads with a Keyblade instead of a simple sword. A Keyblade has mystic powers beyond one world and it would make some bit of sense.

Trivia

 * The Hydra is based on the Greek Mythogical monster: Hydra.
 * The Hydra is rendered in CGI, unlike the rest of the movie. This was done as it would have been incredibly difficult to draw each head individually. This was actually the first time Disney used CG for anything.
 * According to the mythology, the only way to kill the Hydra from growing new heads is to slice each heads and then torch to stop growing new heads.