Hydra, the sea serpent:

Hydra is the largest and the longest constellation in the night sky. The stars that make up Hydra represent the sea serpent that Hercules was forced to defeat as one of his Twelve Labors. Hydra had nine heads, one of which was immortal. Whenever he cut off one head, two would grow back in its place. He solved this problem by burning the stump of each head that he cut off, thereby stopping the new ones from growing. He then defeated the immortal head by burying it under stones were it could not harm anyone. The constellation of Hydra contains only one bright star Alphard, known as the "Solitary One", since it is the only bright star in that part of the sky. The constellation can be found to the south of Leo, the lion.