Scylla (sea monster)

In ancient Greek mythology, Scylla was female monster with legs made of snakes, with six heads and upper bodies. Her food? Humans, according to Elizabeth Vandiver, Classics scholar and authority on Greek mythology and Greek tragedy including the Iliad, Odyssey, Aeneid, Homer, and Virgil. This definition is based on her course Classical Mythology from The Teaching Company.

Origin myth

Thomas Bulfinch, in his mythology book The Age of Fable, retells the account of Scylla's creation as a monster as told in Ovid's Metamorphoses. Scylla was a beautiful woman beloved by the sea god Glaucus. But Scylla spurned the affections of Glaucus so that he sought a love potion from the sorceress Circe. The sorceress, instead of helping Glaucus win the affections of Scylla, turned her into a sea nymph who grew into a monster and eventually morphed into a dangerous rock on the Sicilian coast, facing the whirlpool Charybdis. Many mariners were reputed to have been wrecked between the two sea hazards. The following account of Scylla's creation is from Bulfinch::

Glaucus said to Circe: “Goddess, I entreat your pity; you alone can relieve the pain I
suffer…I love Scylla.  I am ashamed to tell you how I have sued and promised to her, and 
how scornfully she has treated me.  I beseech you to use your incantations, or potent 
herbs, if they are more prevailing, not to cure me of my love,--for that I do not wish,--
but to make her share it and yield me a like return.”

To which Circe replied, for she was not insensible to the attractions of the sea-green deity, 
“You had better pursue a willing object; you are worthy to be sought, instead of having to 
seek in vain.  Be not diffident, know your own worth.  I protest to you that even I, 
goddess though I be…should not know how to refuse you.  If she scorns you scorn her; meet 
one who is ready to meet you half way, and thus make a due return to both at once.”

To these words Glaucus replied: “Sooner shall trees grow at the bottom of the ocean, and 
the sea weed at the top of the mountains, than I will cease to love Scylla, and her alone.”

The goddess was indignant, but she could not punish him, neither did she wish to do so, 
for she liked him too well; so she turned all her wrath against her rival, poor Scylla.

[Jumping ahead to the next time Scylla bathed in the sea, due to Circe’s powers:]  The lower 
half of Scylla’s body was turned into a bunch of writhing sea serpents and barking monsters, 
still attached to her body.  Scylla remained rooted to the spot.  Her temper grew as ugly 
as her form, and she took pleasure in devouring hapless mariners who came within her grasp…
till at last she was turned into a rock, and as such still continues to be a terror to mariners.

Ovid's account of Scylla being turned into a rock

Ovid also describes how Scylla ends up as a rock:

Glaucus, {still} in love, bewailed {her}, and fled from an alliance with
Circe, who had {thus} too hostilely employed the potency of herbs.
Scylla remained on that spot; and, at the first moment that an
opportunity was given, in her hatred of Circe, she deprived Ulysses of
his companions. Soon after, the same {Scylla} would have overwhelmed the
Trojan ships, had she not been first transformed into a rock, which even
now is prominent with its crags; {this} rock the sailor, too, avoids.

Description of Scylla; and her companion monster Charybdis

Metamorphoses translator Henry Riley's commentary on Scylla the rock's location and danger, along with a neighboring hazard called Charybdis:

 According to some authors, Scylla was the daughter of Phorcys and
 Hecate; but as other writers say, of Typhon. Homer describes her in
 the following terms:-- ‘She had a voice like that of a young whelp;
 no man, not even a God, could behold her without horror. She had
 twelve feet, six long necks, and at the end of each a monstrous
 head, whose mouth was provided with a triple row of teeth.’ Another
 ancient writer says, that these heads were those of an insect,
 a dog, a lion, a whale, a Gorgon, and a human being. Virgil has in a
 great measure followed the description given by Homer. Between
 Messina and Reggio there is a narrow strait, where high crags
 project into the sea on each side. The part on the Sicilian side was
 called Charybdis, and that on the Italian shore was named Scylla.
 This spot has ever been famous for its dangerous whirlpools, and the
 extreme difficulty of its navigation. Several rapid currents meeting
 there, and the tide running through the strait with great
 impetuosity, the sea sends forth a dismal noise, not unlike that of
 the howling or barking of dogs, as Virgil has expressed it, in the
 words, ‘Multis circum latrantibus undis.’