The Hydra which lived in the swamps near to the ancient city of Lerna in Argolis, was a terrifying monster which like the Nemean lion was the offspring of Echidna (half maiden - half serpent), and Typhon (had 100 heads), other versions think that the Hydra was the offspring of Styx and the Titan Pallas. The Hydra had the body of a serpent and many heads (the number of heads deviates from five up to one hundred there are many versions but generally nine is accepted as standard), of which one could never be harmed by any weapon, and if any of the other heads were severed another would grow in its place (in some versions two would grow). Also the stench from the Hydra's breath was enough to kill man or beast (in other versions it was a deadly venom). When it emerged from the swamp it would attack herds of cattle and local villagers, devouring them with its numerous heads. It totally terrorized the vicinity for many years.
Heracles journeyed to Lake Lerna in a speedy chariot, and with him he took his nephew and charioteer Iolaus, in search of the dreaded Hydra. When they finally reached the Hydras' hiding place, Heracles told Iolaus to stay with the horses while he drew the monster from its hole with flaming arrows. This brought out the hideous beast. Heracles courageously attacked the beast, flaying at each head with his sword, (in some versions a scythe) but he soon realized that as one head was severed another grew in its place. Heracles called for help from Iolaus, telling him to bring a flaming torch, and as Heracles cut off the heads one by one from the Hydra, Iolaus cauterized the open wounds with the torch preventing them from growing again. As Heracles fought the writhing monster he was almost stifled by its obnoxious breath, but eventually, with the help of Iolaus, Heracles removed all but one of the Hydras' heads. The one remaining could not be harmed by any weapon, so, picking up his hefty club Heracles crushed it with one mighty blow, he then tore off the head with his bare hands and quickly buried it deep in the ground, placing a huge boulder on the top. After he had killed the Hydra, Heracles dipped the tips of his arrows into the Hydras' blood, which was extremely poisonous, making them deadly.
Other versions say that while Heracles fought the Hydra the goddess Hera sent down a giant crab which attacked his feet). This legend comes from a marble relief dating from the 2nd century BCE found at ancient Lerna, showing Heracles attacking the Hydra, and near his feet is a huge crab. Also other legends say that a stray arrow set alight the forest, and it was the burning trunks which Heracles ripped up and used to cauterize the open wounds.
One of the Gorgons, and the only one who was mortal. Her gaze could turn whoever she looked upon to stone. There is a particular myth in which Medusa was originally a beautiful maiden. She desecrated Athena's temple by lying there with Poseidon. Outraged, Athena turned Medusa's hair into living snakes.
Medusa was killed by the hero Perseus with the help of Athena and Hermes. He killed her by cutting of her head and gave it to Athena, who placed it in the center of her Aegis, which she wore over her breastplate.
From Medusa's dead body the giant Chrysaor and the winged horse Pegasus, her son by Poseidon, sprang forth.
The story of Perseus and Medusa, as told by Thomas Bulfinch.