The Trojan Sea Monster (also known as Ketos Troi) is a monster that Poseidon sent to punished the city of Troy during the rule of Laomedon.
It was sent by Poseidon as revenge against the Trojan king, Laomedon, for cheating the god out of payment for building the walls of Troy. It caused a lot of trouble for the Trojans. Laomedon decided to chain his daughter, Hesione for the monster, as a sacrifice to appease Poseidon.
Hercules slew the monster by allowing himself to be swallowed, and stabbed it from within. Laomedon promptly went back on his word and drove Hercules away. Years later, Hercules, Telamon and Peleus returned to exact revenge. Telamon knew the weakness in the Trojan walls and exploited it. Hercules sacked the town and slaughtered Laomedon and his sons, except Priam. Hesione begged Hercules to spare Priam's life, and Hercules gave her in marriage to Telamon.
While speaking with Porphyrion, Jason Grace says he was the slayer of the sea monster of Troy. Also, Aeolus mentions that Jason has already been to his palace once when searching for information on a sea monster.