Hydra

Hydra was an ancient nameless serpent-like chthonic water beast (as its name evinces) that possessed many heads — the poets mention more heads than the vase-painters could paint, and for each head cut off it grew two more — and poisonous breath so virulent even her tracks were deadly in Greek mythology.

History
After Hera finds out Zeus has an illegitimate son called, she sets Hercules 12 labors to complete. The first is to kill the Hydra, which grows two heads for everyone cut off. Hercules defeats it by making it argue over who will rob him.