The Greeks believed the sun travels under the ocean at night. This makes sense when you consider that the sun seems to drop into the water when you are standing on the seashore facing west. The god of the sea, Poseidon (Roman name Neptune), owns a set of horses that pull the sun through the water, the counterparts of the horses who pull the sun through the sky. These horses can be spotted near the shore occasionally, their manes twirling in white-cap waves, their feet running onto the sand.
Poseidon the water-horse god has a persistent, sometimes violent rivalry with the goddess Athena, whose sacred olive tree grows in country bordering the sea. The waves constantly batter and erode the shoreline as Poseidon seeks to expand his territory.
Poseidon the horse god was a late-comer to the Aegean who “married” a sea goddess called Amphitrite in a patriarchal takeover of an older cult. Amphitrite, who does have a querulous side like Poseidon, is the personification of the sea and a mother goddess of animals.