Glaucus, in Greek mythology, was a sea deity whose origins differ in different sources. His best known story is the one told by Ovid.
In Euripides' play Orestes, Glaucus was the son of Nereus, having helped Menelaus and the Argonauts in their journeys.
Athenaeus of Náucratis (a contemporary of the emperor Marcus Aurelius), in his book The Banquet of the Scholars, after mentioning a fish called glaucus, summarizes the various versions of Glaucus' origin.
According to this version, Glaucus was born a mortal, living as a fisherman. Accidentally discovering a magical herb that could bring the fish he caught back to life, he decided to try it on himself. After eating the herb, Glaucus also jumped into the water, following the example of the fish that revived.
He was welcomed by the water deities, and with the consent of the gods Ocean and Thetis, he was washed of all that was mortal, becoming immortal, and having his form changed: his hair became green like the sea, his shoulders widened, and his legs became a fish tail.
Glaucus fell in love with Scylla, a beautiful nymph, but was unrequited: the nymph always turned away from the water when he tried to approach, frightened by his appearance. The god then turned to the sorceress Circe for help, asking her to prepare a potion that would make Scylla love him.
However, since the witch herself was already in love with him, she tried to dissuade him with affectionate words. He, however, claimed that trees would grow at the bottom of the sea and seaweed on top of the mountains before he would stop loving Scylla.
Enraged, Circe bewitched the fountain in which Scylla bathed, so that Scylla became a twelve-foot, six-headed monster. So when Scylla finally entered the fountain, she saw snakes in the water.
Trying to escape, she realized that the serpents were actually her own body. The nymph went crying to Glaucus, who refused her because of her terrible appearance. Circe waited for the god's visit, but he, knowing it was her doing, did not forgive her for her cruelty.
Glaucus still tried for the love of Ariadne, abandoned by Theseus on the island of Naxos, but, he did not get her either, for Dionysus betrothed her.