A Hydra is a serpent-like aquatic creature with multiple heads. If one head is cut off, it grows back. It has sharp claws and teeth and poisonous breath. It uses it's two tails to swim.
Hercules defeated one, the Lernaean Hydra, as one of his Twelve Labours by cutting off each head (except the immortal one) and scorching the stumps, to prevent any heads from growing back. He then placed the immortal head under a large rock.