ENCYCLOPEDIA
HYDRA. This monster, like the lion, was the offspring of Typhon and Echidna, and was brought up by Hera. It ravaged the country of Lernae near Argos, and dwelt in a swamp near the well of Amymone: it was formidable by its nine heads, the middle of which was immortal. Heracles, with burning arrows, hunted up the monster, and with his club or a sickle he cut off its heads; but in the place of the head he cut off, two new ones grew forth each time, and a gigantic crab came to the assistance of the hydra, and wounded Heracles. However, with the assistance of his faithful servant Iolaus, he burned away the heads of the hydra, and buried the ninth or immortal one under a huge rock. Having thus conquered the monster, he poisoned his arrows with its bile, whence the wounds inflicted by them became incurable. Eurystheus declared the victory unlawful, as Heracles had won it with the aid of Iolaus. (Hes. Theog. 313, &c.; Apollod. ii. 5. § 2; Diod. iv. 11; Eurip. Herc. Fur. 419, 1188, Ion, 192; Ov. Met. ix. 70; Virg. Aen. viii. 300; Paus. ii. 36. § 6, 37. § 4, v. 5. § 5; Hygin. Fab. 30.)
Source: Dictionary of Greek and Roman Biography and Mythology.