The Lernaean Hydra

The Lernaean Hydra
In Greek mythology, The Lernaean Hydra (C.C. not known, in Greek for “water serpent” ) was an ancient nameless serpent-like chthonic water beast that possessed numerous heads—the poets mention more heads than the vase-painters could paint—and poisonous breath (Hyginus, 30). 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, for Lerna was the site of the myth of the Danaids. Beneath the waters was an entrance to the Underworld, and the Hydra its guardian (Kerenyi 1959, p. 143). (Note: do not confuse this with the other guardian of the Underworld, Cerberus.)
The Hydra was the offspring of Typhon (had 100 heads) and Echidna (half maiden-half serpent), noisome creatures of the Goddess, who became Hera. It was said to be the sibling of the Nemean Lion, yet another creature of the archaic Goddess, and thus seeking revenge for Heracles' slaying of it. As such, it was said to have been chosen as a task for Heracles so that Heracles would likely die.
Upon reaching the swamp near Lake Lerna, where the Hydra dwelt, Heracles (as known as Hercules) covered his mouth and nose with a cloth to protect himself from the poisonous fumes and fired flaming arrows into its lair, the spring of Amymone, to draw it out. He then confronted it, wielding a harvesting sickle in some early vase-paintings; Ruck and Staples (p. 170) have pointed out that the chthonic creature's reaction was botanical: upon cutting off each of its heads he found that two grew back, an expression of the hopelessness of such a struggle for any but the hero, Heracles.
The details of the confrontation are explicit in Apollodorus (2.5.2): realising that he could not defeat the Hydra in this way, Heracles called on his nephew Iolaus for help. His nephew then came upon the idea (possibly inspired by Athena) of using a burning firebrand to scorch the neck stumps after decapitation, and handed him the blazing brand. Heracles cut off each head and Iolaus burned the open stump leaving the hydra dead; its one immortal head Heracles placed under a great rock on the sacred way between Lerna and Elaius (Kerenyi1959 p 144), and dipped his arrows in the Hydra's poisonous blood, and so his second task was complete.
I would like to point out that in different sources they state that the hydra has from seven to nine heads (there was the rumor of the hydra having fifty heads or even a hundred; maybe because of its ability to replace one head with two). In many stories they tell that the hydra had only seven and there have been other cases, like this one, where the hydra has nine heads. But until today Hydra is considered to be a Seven-Headed Dragon. It is also believed that one of the seven (or nine) heads was made of gold and immortal. That head was the one, at being cut, it would not become two heads but instead kill instantanously the beast (why is then immortal, I do not know.)

Gotten from: Lernaean Hydra, Wikipedia.
The Book of Dragons and other Mythological Beasts, Joseph Nigg.
Dragon a Natural History, Dr. Karl Shuker.