Poseidon was the Ancient Greek god of the sea, the Romans called him Neptune. Poseidon was a son of Chronos brother to Zeus and the second most powerful of the pantheon. Poseidon was one of those who Got swallowed alive and survived.
Poseidon was a person, he fell in love with Demeter and created a horse to impress her. Alternatively Demeter changed herself into a mare to escape Poseidon and Poseidon turned himself into a stallion to capture her. A horse, Arion was born from this union. 
When Poseidon was in a good mood the sea was calm but Poseidon was frequently angry and at such times he stirred the sea up with his trident creating storms or stirred up the ground creating earthquakes.  
This is yet another example of ridiculous things people believe as part of a religion.