The hydra, also known as the Lernaean Hydra, is a work of Greek mythology. It is a serpent-like creature with many heads. It is the offspring of Typhon and Echidna and the siblings of Nemean Lion, the Chimena, and the Cerberus[1]. It dwelled in the swamps of Lerna in Argolis.
It killed by the horrible smell of its breath. It would also attack unexpectedly from the dark depths of the swamp it lived in, and would devour anything that came too close. The Hydra could not be killed by any weapon because whenever one of its heads was cut off, two would grow in its place.

Origin: Hercules and his nephew Iolaus went to go battle with the Hydra. After luring it out of the swamp, he attempted to kill it. He soon came to realize, the Hydra could not be harmed by cutting its numerous heads off. He came up with a plan to cauterize the wounds thus making it immpossible for more to grow back in their place. When all but a single head remained, Hercules crushed it with his club, and burried it deeply in the ground, placing a huge boulder on top of it[2].

Appearance: The Hydra is a lage creature with many heads. Most accounts say that it has nine heads, but some say that number could have been six, seven, eight, or even fifty[3]. It is said to have the body of a serpent with a very long tail.

Influences on the Modern Day: In 1997, Disney produced the movie, Hercules. In Hercules, the character Hades is sent to kill the Hydra, but he prevails[4]
[1]Wikipedia.org: Lernaean Hydra
[2]Pantheon.org: “Hydra” by Ron Leadbetter
[3]Monsters.monstrous.com: Hydra
[4]Wikipedia.org: Hercules (1997 film)