Poseidon was the son of Cronus, the Titan, who swallowed him and his brother and sisters whole. Zeus, the king of the gods, slew Cronus and set Poseidon free.
Though Poseidon pursued and raped many women, he was married to the water nymph Amphitrite. Their son was the god Triton.
Poseidon's sway over horses began when he pursued Demeter, the goddess of nature. She turned into a mare to flee him and he, in turn, became a stallion to chase her.
Poseidon competed with the goddess Athena for stewardship of Athens. He created a saltwater fountain for the city, but it was deemed less useful than the olive tree Athena planted.
When Odysseus was returning home from the Trojan War, he blinded the cyclops Polyphemus--Poseidon's son. The angry god then cursed Odysseus and it took 10 more years for the poor man to reach home.