Lerna, a lake that could be used as one of the entrances to the underworld, Hades posted HYDRA, another child of Tartarus and Gaia. I mentioned before the frightening mutations possible when monsters mate, and the difference between Cerberus and his sister Hydra offers a striking example. On the one hand, a dog with a more or less manageable three heads and an elegantly snaky tail to wag; and on the other, his sister, a many-headed water-beast who was almost impossible to kill. Chop off one of her heads and she could grow back ten more in its place.
