Lernaean Hydra

The Hydra is a monstrous, multi-headed reptile from the myths of Ancient Greece. It made its lair in Lake Lerna. According to the myth, beneath the lake was an entrance to the Underworld, and the Hydra was it's guardian. Most of the time, it stayed in the spring of Amymone, a deep cave, only coming out to terrorize neighboring villages. Believed by some to be a type of dragon, the Hydra would meet its end at the hands of Heracles, as the second of his Twelve Labours.

Appearance
Most myths agree that the hydra is a sea monster with many heads (according to the poets, more heads than the vase-painters could paint), one of which was immortal. Common depictions of the hydra give the creature a thick, strong body, a serpentine tail and webbed feet.

Powers

 * Immortality - The largest of the Hydra's head is immortal, and cannot be damaged by conventional weapons. The only way Heracles was able to kill this head was to cut it off with a golden sword, gifted to him by Athena.
 * Regeneration - With all other heads, the Hydra would grow two for every one that was cut off. This gave it a theoretically infinite amount of heads. However, this regeneration could be cancelled out if someone were to cauterize the wound.
 * Poisoned Blood - The blood of the Hydra was known to be a powerful poison.
 * Poisonous Breath - Some variations of the myth give the Hydra the ability to breath a cloud of poisonous gases.