Tros' son, Ganymede, was kidnapped by Zeus, who had fallen in love with the beautiful boy.
Tros grieved for his son. Sympathetic, Zeus sent Hermes with two horses so swift they could run over water. Hermes also assured Tros that Ganymede was immortal and would be the cupbearer for the gods, a position of much distinction.
Before the Trojan War, Poseidon sent a sea monster to attack Troy. 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' son. Tros agreed; Heracles succeeded and Telamon married Hesione, Tros' daughter, giving birth to Teucer by him.
Note: In an alternate version, Tros is replaced by Laomedon, who sought to sacrifice Hesione to appease Poseidon. He is killed, along with the monster, by Heracles.