A Hydra is a legendary beast from Greek and Roman mythology. It had multiple heads, one two for every time one was cut off. The Lernean Hydra was the offspring of Echidna and Typhon, and was slain by Hercules.
The Lernean Hydra was raised by Hera to kill Hercules, but the water beast was killed by the demigod. Hera was upset by this, and placed the Hydra in a dark vault in the sky to become the Constallation Hydra, shaped as a long water snake.