Hydra

"Will you forget the head-slicing thing!"

- Phil during Hercules's fight with the Hydra.

The Hydra is a large, multi-headed serpent 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 appearance
The Hydra is a massive, dragon-like reptilian creature with dark purple skin. Its body is rather bulky and it crawls using 2 legs with 4-fingered hands that have white claws. It has a long, thin tail and each time one of its heads gets cut off, 3 more grow to replace it. Its number of heads can apparently grow exponentially up to at least 18 in Hercules, though in Kingdom Hearts II its maximum is 7 and in the read-along story it was 30. All of the Hydra’s heads are identical to each other, each resembles a cross between a snake's and a monkey's and sporting the same long, serpentine neck, a 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.

Hercules
The Hydra is the first obstacle sent by Hades to kill Hercules. It is trapped in a cave under a boulder until Hercules unknowingly releases it 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 defenestrates the beast by hurling a boulder into its mouth, but it only crushes it into stones and laughs. When the monster next strikes, he holds its mouth open and slams it head into the ground, giving him time to grab his sword. However, the Hydra wraps its tongue around his ankle, hurls him into the air, and swallows him. While in the monster's throat, Hercules decapitates it 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. It 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, continuing until he is surrounded by an army of heads.

Hercules is knocked off Pegasus and falls into the the monster's forest of heads, he barely makes it through before he is sent flying the air. The Hydra then grabs him and as it is about to finish the young hero off, Hercules notices a rocky cliff that he quickly punches, which causes an avalanche of rocks to fall. The Hydra freezes as all of its heads look up in fear, when it and Hercules are soon buried and crushed, ultimately defeating the Hydra for good.

All that is seen of the Hydra after the rockslide is its right arm and hand (where Hercules was held). Everyone, including Pegasus, Phil, and Hades thinks that Hercules had died with it; however, all a sudden the Hydra's arm started moving. The people are afraid at first, thinking that the Hydra is still alive, until it's revealed to be none other then Hercules who survived, everyone soon started cheering for Hercules as the light shine on a new hero.

During the song, Zero the Hero, everyone is watching a play of how Hercules bravely fought the Hydra in battle.

Hercules: The Animated Series
The Hydra only made minor appearance in the series. In the episode "Hercules and the Parent's 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 Arabian Night, Hades thinks about where he can put the Hydra when he meets Jafar for the first time. We can see also a little figure of the Hydra in this sequence.

In Hercules and the First Day of School, the Hydra head was seen in the museum.

In Hercules and the Gorgon, the Hydra is invited by Pain and Panic to the orientation in the Underworld.

House of Mouse
The Hydra appears briefly in the Philoctetes' Hero-Training Program advertisement at the end of the episode "Salute To Sports".

Kingdom Hearts II
The Hydra appears in Kingdom Hearts II as 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.

Trivia

 * The Hydra is based on 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 the second part of his Twelve Labors after The Nemean Lion.
 * When the Hydra was seen as a baby being fed by Echidna, it already has three heads even though it started with only one.
 * As stated in the myth, Hercules slew the Hydra by slicing off its heads, then using a torch to cauterize the stumps left.
 * The Hydra is rendered in CGI, unlike the rest of the movie which is in hand-drawn animation. This was done as it would have been incredibly difficult to draw each head individually.
 * The boulder that Hercules threw at the Hydra was shaped like a blaster.
 * When the Hydra bit the boulder in half, it's laugh was the grunting of a hippopotamus.
 * The Hydra is a rare instance of blood and gore shown onscreen in a Disney movie, as it visibly bleeds green blood after its heads are cut off.
 * In Once Upon a Time, the Hydra does not appear. However, when Hercules tells his twelve labors to a teenage Snow-White, one of his medals represent the creature.
 * According to the Disney Read-Along story of Hercules, the Hydra grew a total of 30 heads.
 * However, the math shows if Hercules removed 1 head and 3 more were added up to 14 times, the closest result would be 29.