Big O

The Hydra seems to be a creation of Eugene, formed from the remains of the "Wraithful Titan" (see episode three, "Electric City") salvaged by Rosewater's people. Like it's original, it's a giant electric eel creature, but is albino and has two additional heads with regenerative powers so that it can absorbed three-times the amount of electricity and three times the electric shock. It was invented to fulfill its "parent's" original purpose to provided the city with energy.