Before the Trojan War, Poseidon sent a sea monster to attack Troy.
In the King Tros version, Heracles (along with Telamon and Oicles) agreed to kill the monster if Tros would give him the horses he received from Zeus as compensation for Zeus' kidnapping Ganymede. Tros agreed; Heracles succeeded and Telamon married Hesione, giving birth to Teucer by him.
In the King Laomedon version, Laomedon planned on sacrificing Hesione to Poseidon in the hope of appeasing him. Heracles rescued her at the last minute and killed both the monster and Laomedon and Laomedon's sons, save Ganymede, who was on Mt. Olympus, and Podarge, who saved his own life by giving Heracles a golden veil Hesione had made. Telamon took Hesione as a war prize and she gave birth to Teucer by him.