Achaeus (son of Poseidon)In Greek mythology, Achaeus or Achaios was the eponym of Achaea.FamilyAchaeus was the son of Poseidon, the god of the sea and Larissa, daughter of Pelasgus, the son of Triopas, meaning he is of Argive descent through his mother's parentage. He is the brother of Phthius and Pelasgus.MythologyTogether with his brothers Phthius and Pelasgus, they left Achaean Argos with a Pelasgian contingent for Thessaly. They then established a colony on the said country naming it after themselves. The only source of the accounts of Achaeus is recounted by Dionysius of Halicarnassus in his Roman Antiquities about the Pelasgian race's migration in connection with Achaeus.