Homer's Odyssey gives Scylla no real backstory, presenting her simply as a monster already fixed in her cave, but later Greek and Roman tradition, most fully in Ovid's Metamorphoses, supplies one: Scylla begins as a beautiful sea nymph, desired by the sea god Glaucus, who seeks a love potion from the sorceress Circe to win her. Circe, jealous, poisons the pool where Scylla bathes instead, transforming her lower body into a ring of ravening dog heads and turning the nymph permanently into the monster later sailors would fear. Ovid's telling turns Homer's already-monstrous hazard into a tragedy with a clear villain in Circe and a victim in Scylla, a shift consistent with a wider Roman literary habit of supplying emotionally engaging origin stories for monsters Greek epic had simply presented as given features of the world.
Whatever the two poets' differences in how Scylla came to be, the pairing with Charybdis endured into English as a durable idiom, "between Scylla and Charybdis," describing exactly the situation Odysseus faced: forced to choose between two dangers where avoiding one necessarily means accepting exposure to the other, with no route that avoids both. The idiom's survival across roughly three thousand years of continuous use, from Homer's original audience through to ordinary modern English, is itself a measure of how precisely the image captures a genuinely common kind of difficult choice, one with no safe third option available.