It's true her voice sounds like a new-born pup,
but she's a vicious monster.  Nobody
would feel good seeing her, nor would a god
who crossed her path.  She has a dozen feet,
all deformed, six enormously long necks,
with a horrific head on each of them,
and three rows of teeth packed close together,
full of murky death.  Her lower body
she keeps out of sight in her hollow cave,
but sticks her heads outside the fearful hole,
and fishes there, scouring around the rock
for dolphins, sword fish, or some bigger prey,
whatever she can seize of all those beasts
moaning Amphitrite keeps nourishing
in numbers past all counting.  No sailors  
can yet boast they and their ship sailed past her
without getting hurt.  Each of Scylla's heads
carries off a man, snatching him away  
right off the dark-prowed ship. The Odyssey: Book 12

Scylla, Louvre CA1341

In Greek mythology, Scylla, or Skylla (Greek Σκύλλα) was a name shared by two beings:

Scylla was the Princess of Megara, daughter of King Nisus who was invincible as long as a lock of red hair still existed, hidden in his white hair. King Minos of Crete attacked Megara but Nisus knew he could not be beaten because he still had his lock of red hair. His daughter, Scylla, fell in love with Minos and proved it by cutting the red hair off her father's head. Nisus died and Megara fell to Crete. Minos killed Scylla for disobeying her father. She was changed into a seabird, relentlessly pursued by her father, who was a sea eagle.

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Scylla Carving from Milos, 5th century BC British Museum, London

Scylla, the daughter of Crataeis, is one of the two sea monsters in Greek mythology (the other being Charybdis) which lives on one side of a narrow channel of water. The two sides of the strait are within an arrow's range of each other, so close that sailors attempting to avoid Charybdis will pass too close to Scylla and vice versa. The phrase between Scylla and Charybdis has come to mean being in a state where one is between two dangers and moving away from one will cause you to be in danger from the other. Traditionally the aforementioned strait has been associated with the Strait of Messina between Italy and Sicily but more recently this theory has been challenged and the alternative location of Cape Skilla in north west Greece suggested. Scylla has the face and torso of a woman, but from her flanks grow six dog heads and her body below that sprouts twelve canine legs and a fish's tail. She was one of the children of Phorcys and Ceto known as the Phorcydes.

She's not human,
but a destroyer who will never die—
fearful, difficult, and fierce—not someone 
you can fight.  There's no defence against her.
The bravest thing to do is run away. 
If you linger by the cliff to arm yourself,
I fear she'll jump out once more, attack you
with all her heads and snatch away six men,
just as before.  Row on quickly past her,
as hard as you can go.  Send out a call
to Crataiis, her mother, who bore her
to menace human beings.  Homer, Odyssey, Book 12

In Homer's Odyssey, Odysseus successfully navigates his ship past Scylla and Charybdis.