Glaucus was a Greek sea-god, the son of Anthedon and Alcyone. The story of his origin is told by Ovid, that he began as a mortal fisherman living in the Boeotian city of Anthedon and one day he caught and landed some fish at a place where there grew a herb with the magic property of resuscitating fish and allowing them to return to the water. Seeing this effect the herb had on the fish, Glaucus ate some of it too. The herb made him immortal, but it also gave him fins and caused his legs to transform into a fish's tail, forcing him to dwell forever in the sea. Glaucus was initially upset by this side-effect, but Oceanus and Tethys received him well and he was quickly accepted among the deities of the sea, learning the art of prophecy at which they were skilled.
Glaucus later fell in love with the Italian maiden Scylla, who rejected him due to his piscine form. He consulted with Circe for a solution but she became passionately in love with him herself. Since Glaucus cared only for Scylla, however, Circe turned her into a fishlike monster from the waist down, with a row of vicious dog's heads round her loins. She went to live alone in a submerged cave overlooking a narrow channel of water, but Glaucus remained in love with her and mourned her transformation.
Euripides wrote in his play Orestes that Glaucus was a son of Nerus and says that he assisted Menelaus on his homeward journey with good advice. He also helped the Argonauts. It was believed that he commonly came to the rescue of sailors in storms, having once been one himself.
Glaucus was a Corinthianian king, son of Merope and Sisyphus. He angered Aphrodite and she made her horses angry during the funeral games of King Pelias. They tore him apart. His ghost supposedly frightened horses during the Isthmian Games.
Iliad II, 876; VI, 199
One day, Glaucus was playing with a ball or mouse and suddenly disappeared. His parents went to the Oracle at Delphi who told them "A marvelous creature has been born amongst you: whoever finds the true likeness for this creature will also find the child."
They interpreted this to refer to a newborn calf in Minos' herd. Three times a day, the calf changed color from white to red to black. Polyidus observed the similarity to the ripening of the fruit of the blackberry plant and Minos sent him to search for Glaucus.
Searching for Glaucus, Polyidus saw an owl driving bees away from a wine-cellar in Minos' palace. Inside the wine-cellar was a cask of honey, with Glaucus dead inside. Minos demanded Glaucus be brought back to life, though Polyidus objected. As Minos hugged his son's corpse, a snake appeared nearby; Polyidus killed it with Minos' sword. Another snake came for the first, and after seeing the dead snake, the second serpent left and brought back an herb which then brought the first snake back to life.
Minos refused to let Polyidus leave Crete until he taught Glaucus everything he knew. Polyidus did so, but then, at the last second before leaving, he asked Glaucus to spit in his mouth. Glaucus did so, giving Polyidus back everything he had been taught.
Glaucus later led an army that attacked Italy, introducing to them the military girdle and shield. This was the source of his Italian name, Labicus, meaning "girdled".