Scylla

Scylla was a sea monster in Greek mythology. She was said to be the daughter of Poseidon. Scylla was a water dwelling version of the Hydra and the sister of Charybdis.
Some myths explain Scylla's beginnings. One of them says she was one of the daughters of Phorcys the Greek God of the Deep. Another says that Scylla was a very pretty fairy who was liked by Poseidon, but the jealous Amphitrite turned her into a monster by poisoning the water of the place where Scylla would swim.
One says that a jealous Circe poured a potion into the sea water and then made Scylla transform into a monster with four eyes and six long necks with ugly heads. Her body had 12 tentacle looking legs and a tail. She attacked the ships of nearby people.
In another myth, Heracles (also known as Hercules) came across Scylla when traveling and then killed her. The sea-god Phorcys, then restored her to life.