In Greek mythology, Cretheus was an Aeolid king, son of Aeolus and founder of Iolcus in Thessaly. By his niece Tyro he fathered Aeson, Amythaon, and Pheres—and thus became grandfather of Jason, so the Argonaut voyage springs from his house. Homer already names the marriage in the Odyssey's catalogue of heroines.
Adapted from Wikipedia, "Cretheus" · CC BY-SA 4.0
I.Aeolid founder of Iolcus
Cretheus was one of the sons of Aeolus son of Hellen, by Enarete (or Laodice in a scholium)—brother to Sisyphus, Athamas, Salmoneus, Deion, Magnes, and Perieres. He founded Iolcus on the Pagasaean Gulf and married Tyro, daughter of Salmoneus.
By Tyro he had Aeson, Amythaon, and Pheres. Homer has Tyro tell Odysseus in the underworld that after bearing Pelias and Neleus to Poseidon she became Cretheus' wife and bore those three sons. Amythaon's line yields the seers Melampus and Bias; Pheres founds Pherae.
II.Jason's grandfather
Through Aeson, Cretheus is grandfather of Jason. Apollonius anchors Jason's kinship claim on Cretheus: Jason son of Aeson, whom Cretheus begat, is paternal kin to the house of Athamas and Phrixus when the Argonauts reach Colchis.
Some later notices add other wives (Demodice or Biadice) and daughters such as Hippolyte (Astydamia), wife of Acastus. The Iolcus kingship later passes under Pelias, Tyro's son by Poseidon, setting the stage for Jason's exile and return.