Which multi-headed serpent grew two heads for every one cut off?

The story behind the answer

Hydra is correct because, in Greek myth, the Lernaean Hydra was a swamp-dwelling serpent that sprouted two new heads for every one cut off. As part of his second Labor, Heracles defeated it by cauterizing the neck stumps with the help of his nephew Iolaus, and he buried the Hydra’s immortal head under a heavy rock.

The Hydra’s name evokes water (from Greek “hydor”), fitting its marshy lair, and “hydra-headed” now describes problems that multiply when attacked. For contrast: the Chimera was a lion-goat-snake composite, the Naga is a South Asian serpent spirit, and Leviathan is a Biblical sea monster. Memory tip: H in Hydra for “Heads” that multiply, and think of the tiny real-life “hydra” animal famous for regeneration.