Eurypylus was the son of Poseidon and Astypalaea and became king of the island of Cos. He was the husband of Clytie and father of Chalciope, Chalcon and Antagoras. Heracles landed on Cos to escape a storm sent upon him by Hera, but the Coans took him for a pirate and attacked him; in a battle that ensued, Eurypylus was killed by Heracles.
In another version, Heracles planned the attack on Cos because he liked Eurypylus' daughter Chalciope and intended to abduct her. Chalciope is indeed known as the mother of Heracles's son Thessalus.