Also calledLernaean Hydra
A scaly monster with nine heads, each one diamond-shaped with a mouth lined with jagged teeth. Each mouth spits poison. If one of its heads is severed, the hydra grows two heads in place of the lost one. . . .
In Greek mythology, the Lernaean 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. The Hydra of Lerna was killed by Heracles as the second of his Twelve Labours. Its lair was the lake of Lerna in the Argolid, though archaeology has borne out the myth that the sacred site was older even than the Mycenaean city of Argos since Lerna was the site of the myth of the Danaids. Beneath the waters was an entrance to the Underworld, and the Hydra was its guardian.
The Hydra was the offspring of Typhon and Echidna (Theogony, 313), both of whom were noisome offspring of the earth goddess Gaia.
Source(s): Percy Jackson and the Olympians - The Ultimate Guide ;;Wikipedia