When did scylla become a monster?

Scylla became a monster in Greek mythology as a result of a curse from the sea goddess Circe. According to the myth, Circe transformed Scylla into a hideous creature after Scylla rejected her advances. This transformation left Scylla with multiple dog heads and a fierce disposition, turning her into a fearsome being that terrorized sailors, particularly in the Strait of Messina, where she is famously depicted in Homer's "Odyssey."