Scylla was once a beautiful girl, a daughter of Apollo and Hecate.
One day, the sea-monster Glaucus (once a mortal fisherman, but became a monster by chewing on a magical herb) fell in love with her. Scylla doubted that he was a monster or a god and so she fled from him, scorning him.
With a broken hart, Glaucus went to Circe to get a magical herb to make Scylla fell in love with him. Unfortunately, Circe herself fall in love with Glaucus. Circe prayed that Glaucus would fall in love with her in stead of Scylla. But Glaucus refused point-black.
Out of jealousy and revenge, the waters wherein Scylla washed her (at the tip of Italy's boot) became enchanted with magical herbs by Circe. Due to that, Scylla morphed into a seamonster with the body of a fish beneath her hips and on those hips, 6 dogs are attached. She also had 12 dog paws.
Ashamed of herself, Scylla stayed at the street of Messina (between Italy and Sicily). There, toghether with Charybdis (who was at the other side of the Street of Messina), she became the nightmare of every sailor:
Those who did want to get swallowed by the whirlpool of Charybdis, had to pass her (and vice versa). She always devoured 6 sailors from a passing ship: grabbing one with each dog-head.
Odysseus and his fleet had to sail through the street of Messina, and they chose Scylla's side: only six men were killed. If they had chosen Charybdis, their whole ship and the sailors would have been sucked down.
When Aeneas passed Scylla, she was already transformed into a rock, who laid there very dangerously. On the other side, the island whereunder Charybdis' cave was, is also an "evidence that she (Charybdis) has "existed".