I do knowIf you really want to get technical about the mythological basis for these Pokemon, consider this: In Greco-Roman mythology, the Hydra and Cerberus were brother and sister, and their mother was a beast known as Echidna.
or at least I should after having written a quadrology of Greek mythology/Japanese folklore based fantasy books. But what's your point? Maybe that we might get more dark types based on the creepy sh*t out of Greek myth? I'm all for that!