Hydra is a character who can be found exclusively as a boss in Kingdom Hearts II.
The Hydra is a massive, reptilian creature with dark purple skin. It has a rather bulky body and 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, 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.
Hades sent the Hydra to destroy the Olympus Coliseum and kill Hercules. Hercules had to fight it in the Coliseum and believed he had defeated it, but it was in fact was merely playing dead and started to destroy the Coliseum.
The Hydra met its match when Sora, Donald, and Goofy as well as Phil, Pegasus, and Meg came in to help out. However, the aftermath of the battle was rather unfavorable—the Coliseum was destroyed, making Hercules to disbelieve himself; thus losing his hero's glow, making him an easy prey for the plans of Hades.
The Hydra originally appeared in the 1997 film Hercules. Hades had sent his two minions, Pain and Panic, to disguise themselves as little boys and pretend that they were trapped under a boulder, so Hercules would help them by removing the boulder, but in truth the Hydra was also hiding behind that boulder. So, after some struggle, Hercules was swallowed by the one-headed Hydra, but he managed to escape by cutting its head off from the inside.
However, the Hydra grew back three heads and continued to attack Hercules. Hercules kept on trying to cut off the Hydra's heads, but soon there were dozens of heads on the Hydra, forcing Hercules to cause a avalanche of rocks to crush him (though he survived) and the beast, slaying the Hydra.
The Hydra's name references the Lernaean Hydra, a serpent-like water monster of Greek mythology.