Mentioned as "Mother Hydra" to "Father Dagon."
Hydra was the many-headed daughter of Echidna and Typhon. She was reared by Hera herself. She lived in the swamps of Lerna. Her name means "water serpent." Hercules killed her by cutting her heads off and having his nephew immediately cauterize the necks before new heads could grow. It is sometimes said she had one head that was eternal which Hercules buried.
[The Shadow Over Innsmouth - H.P.L.]