Polyidus was a descendant of another renowned seer, Melampus. Given that Melampus had two sons, Abas and Mantius, different sources made Coeranus, father of Polyidus, son or grandson of either of the two. Briefly, the two alternate lineages were:
According to a scholiast on Homer' s Iliad, Polyidus had two sons, Euchenor and Cleitus, by Eurydameia, daughter of Phyleus. Pausanias makes Polyeidus father of Coeranus, Manto and Astycrateia, and calls Euchenor his grandson through Coeranus.
The best known myth concerning Polyidus is the one that deals with him saving the life of Glaucus, which runs as follows. One day, Glaucus, son of King Minos and Queen Pasiphaë of Crete, was playing with a mouse and suddenly disappeared. The Kuretes told Minos: "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 mulberry or the blackberry, and Minos sent him to find Glaucus.
Searching for the boy, 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 and shut Polyidus up in the wine-cellar with a sword. When a snake appeared nearby, Polyidus killed it with the sword. Another snake came for the first, and after seeing its mate dead, the second serpent left and returned with an herb which then brought the first snake back to life. With the herb Polyidus resurrected the child.
Minos refused to let Polyidus leave Crete until he taught Glaucus the art of divination. Polyidus did so, but then, at the last second before leaving, he asked Glaucus to spit in his mouth. Glaucus did so and forgot everything he had been taught.