Poseidon - The Greek God

The Greek god of the sea, Poseidon, brother of the gods Zeus and Hades, and the goddesses Hera, Demeter, and Hestia, was associated not just with the sea but with horses. The following is a list of the mates of Poseidon and the children or other offspring they created.

• Amphitrite (a Nereid) Offspring: Triton
• Medusa (the woman whose hair was snakes and who turned men to stone) Offspring: Pegasus (famous flying horse), Chrysaor
• Demeter (or an Erinys or an Harpuia) Offspring: Areion (Adrastos' horse)
• Thoosa (daughter of Phorkys) Offspring: Polyphemus
• Iphimedeia Offspring: The Aloadai (Otos and Ephialtes)
• Tyro Offspring: Neleus and Pelias
• Periboia (daughter of Eurymedon) Offspring: Nausithoos (father of Alkinoos)
• Amymone (the fiftieth Danaid) Offspring: Nauplios
• Pitane Offspring: Euadne
• Euryale Offspring: Orion
• Aithra??? Offspring: Theseus (possibly)

Source: Timothy Gantz' Early Greek Myth

Also see:
Apollo's Mates
Hercules' Mates and Kids
Zeus' Divine Mates