11: The Lernaean Hydra

In Greek mythology, the Lernaean Hydra was a gigantic water-serpent who had nine snake-like heads. This specific Hydra haunted the country of Lerna. Near the well of Amymone, it dwelt in a watered swamp.
Though its parents were Typhoeus and Ekhidna, she was brought up by Hera. The Hydra was the guardian of Lake Lerna, which was supposedly a gate to the Underworld. The Hydra could either kill you by approaching her with her venomous breath or with her poisonous blood. Of the nine heads the serpent had, the middle one was immortal. And to make matters worse, as one head was cut off, two more grew in return.
Eventually, Herakles, along with the help of his cousin, Iolaos, they ended up defeating and killing the Lernaean Hydra. Together, as one head was chopped off, they “applied burning brands to the severed stumps, cauterizing the wounds and preventing regeneration.” During this process, Herakles crushed, beneath his heel, a giant crab which ended up assisting the Hydra. Both were placed among the stars by Hera creating the constellations Hydra and Cancer.