In Greek mythology, the multi-headed serpent slain by Heracles as his second labour is the Lernaean Hydra, usually called the Hydra.
The Hydra was a poisonous lake monster that lived near Lerna in the Argolid. Ancient mythographers describe it as the offspring of Typhon and Echidna, two monstrous figures who also appear in the genealogies of Greek monsters. Its breath and blood were deadly, making the creature dangerous even beyond its many heads.
Heracles discovered that cutting off the Hydra’s heads was not enough: new heads could grow in their place. He therefore called on his nephew Iolaus, who cauterised each neck after Heracles severed it. Heracles buried the monster’s remaining immortal head beneath a great rock and used its poisonous blood on his arrows.
The Hydra is often confused with Echidna, its monstrous mother, or with other serpent creatures such as Python and Ladon. The number of heads also varies by source and artwork, so the canonical identification matters more than a fixed head count.