The Hydra is a local monster, tied to the marsh of Lerna in the Argolid. Hesiod names her a child of Typhon and Echidna, reared by Hera as an adversary for Heracles, whose second labour is her killing. The signature detail, that two heads grow for each one severed, is not in Hesiod and enters the tradition later, along with its solution: cauterising each stump with fire while his nephew Iolaus holds the torch. One head was immortal and was buried under a rock, and because Iolaus helped, Eurystheus refused to count the labour. Apollodorus gives the fullest classical summary. The Hydra embodies the motif of a difficulty that multiplies when attacked, and "hydra-headed" survives as a figure for exactly that; the venom Heracles takes from the carcass later kills him in turn, binding the beginning of his labours to his end.
Multi-headed serpents of this general kind are not unique to Greece. Near Eastern art shows comparable monsters centuries before Hesiod, among them a seven-headed serpent depicted on Old Babylonian cylinder seals, and the Ugaritic texts describing Baal's combat with the serpent Litan, discussed elsewhere in this collection under Leviathan, use language of a many-headed sea-monster that some scholars have connected loosely to the same wider Near Eastern imagery. Whether any direct line of influence runs from these older Near Eastern figures to the specifically Argive Hydra of Lerna cannot be shown from the sources, and the safest statement is that the motif of a monstrous, multiplying, many-headed serpent recurs independently across the ancient Mediterranean world rather than spreading from one single point of origin.