The Hydra, from Greek mythology was, like the Nemean lion, one of the offspring of Echidna and Typhon. Possessing the body of a serpent and a multitude of heads, the Hydra lived in the swamps near to the ancient city of Lerna. As so often happens in myths, the actual details of the Hydra vary; in some accounts it is possessed of a hundred heads, being similar in this respect to its father. In other accounts, the Hydra possesses as few as five. There is, however, a general acceptance that the Hydra, before encountering Heracles, was equipped with nine heads. Advantageously, one of these heads was immortal, and the remaining eight would quickly regrow if severed. Indeed, some versions of the myth specify that the severed stump would regrow two extra heads.
As if this weren't enough, the Hydra's breath was deadly poison in itself, and could quickly do away with most attackers, not to mention its prey. The Hydra would emerge from the swamp and gorge itself on herds of cattle, along with any local townspeople who happened to be in the way.
It is no surprise, then, that more than a few people wanted the Hydra dead and gone. Enter Heracles, sometimes known as Hercules, particularly by Disney and the like. Appollodorus tells us that Heracles was instructed to slay the Lernaian Hydra as his second labour. Driven by Iolaos, his nephew, Heracles travelled to Lerna and sought out the hiding place of the Hydra. Drawing the monster out with flaming spears, Heracles quickly came up against the problem of the regenerating heads. Striking each head with his club produced only more heads, and the Hydra quickly wrapped her coils around Heracles' foot. Things weren't looking good for Heracles, particularly when (in a rather bizarre addition to the story) a giant crab turns up to assist the Hydra.
Crushing the crab, Heracles calls upon Iolaos to make torches and sear the Hydra's regenerating heads, thus keeping them from growing. This strategy worked well, it seems, for Heracles was eventually able to crush and remove the final immortal head, which he chose to bury beneath a large boulder. Never one to waste good venom, we are told that he dips his arrows into the poison for later use.