Amykus was a son of Poseidon and Melia. Amykus was also king of Bebrykia, who forced anyone who came to his kingdom to fight him in boxing match, Amykus, and slew the vanquished similar to how years earlier Kerkyon the king of Elusis forced all travelers to wrestle with him and be killed when they lost. Just like how Theseus in his youth was able to use his skill in wrestling to defeat and kill Kerkyon, so did Polydeukes (Pollux) use his superior skill in boxing to defeat and kill Amykus when he had challenged the Argonauts to a boxing match.
Poseidon & Melia
"Amycus, son of Neptune and Melie, king of Bebrycia, compelled whoever came to his kingdom to contend with him in boxing, and slew the vanquished. When he had challenged the Argonauts to a boxing match, Pollux fought with him and killed him."
- Hyginus, Fabulae, 17