In the Twelve Labors of Hercules, what creature did Hercules defeat in Lerna?

The Lernaean Hydra was the many-headed serpent Hercules defeated in the Twelve Labors.

The Hydra lived near the marshes of Lerna in the Argolid. Ancient stories describe it as a dangerous water serpent descended from Typhon and Echidna. Its most famous feature was its regenerative heads: when Hercules cut off one head, additional heads could grow back.

Hercules brought his nephew Iolaus to help. After Hercules severed each head, Iolaus cauterized the neck with burning brands, preventing new heads from appearing. The central head was immortal in many versions, so Hercules buried it beneath a heavy rock.

Hercules dipped his arrows in the Hydra’s poisonous blood afterward. This detail connects the labor to later episodes, including the death of the centaur Nessus and the fatal shirt associated with Hercules. The Hydra was counted as a failed labor by Eurystheus because Iolaus assisted Hercules.