The hydra was a fearsome nine-headed dragon that inhabited the swamps of Lerna. When one of its' heads got cut off, three new ones grew back in its place. The only way to prevent this was to cauterize the wound before the heads could grow back in. The Hydra of Lerna was killed by the Greek hero Herakles and his nephew, Iolaus.
The name Hydra comes from the Greek word for water.