The Greek myth of Scylla has several versions (as usual, but you already expected that, I hope). But who was she, a horrible monster or just a beautiful girl who fell in love with the wrong guy?
In one of the versions, Scylla was born already a monster, being the daughter of either the sea divinities Phorcys and Ceto (also called Crataeis) (all of their children - the Phorcydes - were monsters; and, with Phorcys being Poseidon's son, she was Poseidon's and Amphitrite's granddaughter), or Phorcys and Lamia, or Triton/Poseidon and Crataeis.
Homer makes Scylla the daughter of Crataeis and describes her as a monster with six long necks with six heads and twelve feet.
When Odysseus left Circe's island, she advised him to sail closer to Scylla than to Charybdis. Scylla had six heads with sharp teeth and she darted from her cave to catch and eat dolphins and sharks. Circe taught Odysseus to ask Scylla's mother, Crataeis, to hold her from darting twice, so as to lose only six of his men (because the six heads of Scylla would each eat one man). Charybdis, instead, would have gulped all of his fleet.
Later, in Latin literature, Pseudo-Hyginus and Ovid make Scylla a beautiful girl who loved to bathe at sea. The marine divinity Glaucus fell in love with her, but when he got out of the water she was afraid and run away. He went to the sorceress Circe, to ask for a love potion. Circe fell in love herself with Glaucus and she tried to woo him, but he only thought of Scylla. That's why the witch became jealous of Scylla and poured a potion in the pool where the poor girl bathed, managing to turn her into a monster.
That's why later, when Scylla ate Odysseus' men, this was considered her revenge against Circe (because the witch had loved Odysseus).
In the later literature, Scylla was described as a beautiful woman, from head to waist. Below the waist she had the tail of a serpent (or the tail of a fish) and six dog heads.
The byzantine scholar John Tzetzes (who lived in the 12th century a.C.) wrote commentaries on the Greek ancient literature. As regards the myth of Scylla, he is the one who said that Scylla was a nymph loved by Poseidon and that's why Amphitrite, who was jealous, turned her into a monster.
As you see, there are many versions about her, the older ones make her a monster by birth (and also, in ancient literature, Amphitrite was quite patient with all her husband's affairs and she was friendly to his children) and the newer ones embrace the idea that in the beginning she was a beautiful girl who was unjustly punished, which makes the myth of Scylla's fate even more tragic.