The many-headed hydra is a real creature that regenerates itself and can live "forever"

Named after the Lernaean Hydra, a many-headed water monster of Greek and Roman mythology, the real hydra is a small fresh-water creature with incredible biological superpowers that enable it to regenerate and live "forever." Unlike the Lernaean Hydra, it doesn't guard an entrance into the underworld. We think. From KQED's Deep Look:

Cut a hydra in half and within a few days the bottom half will grow a new head. The severed head grows a new body. The secret to the hydra's ability to heal and its seeming ability to not age is the abundance of stem cells that make up the hydra's tube-shaped body column. "The reason why hydra lives so long is because it has these continually active stem cells populations which don't seem to slow down at all," said Celina Juliano, a molecular and cellular biologist at UC Davis. "They're just constantly remaking all of the cells in the animal so every 20 days all of the cells are replaced with new cells."