Scylla is a monster encountered by Odysseus during his journey home from the Trojan War. She is often depicted with the upper body of a woman and the lower body of a sea creature, with dog heads sprouting from her midsection.
Scylla was once a beautiful young woman before being transformed into a monster. Details of her parentage vary; sources list Crataeis or Hecate as her mother, and Triton or Phorcus as her father. When Glaucus the sea god fell in love with Scylla, she fled from him in disgust. Glaucus appealed to the nymph Circe to craft a love potion for him, but Circe became jealous and instead made a potion that changed Scylla into a monstrous beast. In Homer's "Odyssey", she's described as having "twelve dangling feet", while in the work of Gaius Julius Hyginus she is said to have twelve tentacle-like legs and four to six dog heads ringing her waist.
Scylla's lair is one on side of a narrow strait, and on the other side lives another sea monster named Charybdis. Sailors attempting to pass the narrow channel would try to avoid one creature, and end up within reach of the other. Odysseus was forced to navigate these waters during his long journey home and lost several of his men to Scylla. The position of these two monsters has given rise to the phrase "between Scylla and Charybdis" to describe choosing between two equally poor options. Variations of this phrase include "between the devil and the deep blue sea" and "between a rock and a hard place".