In Greek mythology, the Lernaean Hydra was the many-headed serpent killed by Heracles during his second labor.
The Hydra lived near the marshes of Lerna in the Argolid and was described as the offspring of Typhon and Echidna. Its poisonous breath and blood made it exceptionally dangerous. Ancient accounts differ about its exact number of heads, but later tradition commonly gives it nine.
Heracles discovered that cutting off a head was not enough because two heads could grow back from one wound. His nephew Iolaus helped by cauterizing each severed neck, preventing regeneration. Heracles then buried the immortal central head beneath a large rock and used the creature’s poisonous blood on his arrows.
The Hydra is often called a dragon in modern summaries, but ancient Greek sources generally describe it as a water serpent. It should not be confused with Ladon, the separate dragon of the Hesperides.