What is the connection between Poseidon and horses?

Question

Since Poseidon is well known as the god of the God of the sea, storms, earthquakes and horses? What is the connection between Poseidon and horses? As a god of the sea the storms and earthquakes part makes sense but where does the part of the horse come in?

Sailors prayed to Poseidon for a safe voyage, sometimes drowning horses as a sacrifice; in this way, according to a fragmentary papyrus, Alexander the Great paused at the Syrian seashore before the climactic battle of Issus, and resorted to prayers, "invoking Poseidon the sea-god, for whom he ordered a four-horse chariot to be cast into the waves." _{Papyrus Oxyrrhincus FGH 148, 44, col. 2; quoted by Robin Lane Fox, Alexander the Great (1973) 1986:168 and note. Alexander also invoked other sea deities: Thetis, mother of his hero Achilles, Nereus and the Nereids}

Like Poseidon, Neptune was worshipped by the Romans also as a god of horses, under the name Neptunus Equester, a patron of horse-racing.

Answer 1

I can't find a classical source for this, but from what I understand it's related to the myth of the founding of Athens and the competition Poseidon had with Athena over who would be the city's patron. They each created various things and the citizens of Athens ended up choosing Athena for her olive tree. In some versions Poseidon gave them a spring of water that turned out to be salty and not good to drink, while in others he gave them the horse, in both cases losing to Athena.

There's also the one where he was in love with Demeter and decided to make the most beautiful animals in the world to woo her, and worked long and hard on it, but by the time he was done he was no longer in love with her. The result was the horse. (I read about this one here)

Wikipedia also has a section about Poseidon's theoretical origins in horse-worship, and discusses (very briefly) the other connections.

Answer 2

His connection is that he created horses. There are different versions of the story, but one of them is that in an attempt to woo Dementer, and so created them out of sea foam. There are also different endings to this, one where Dementer consents and gives herself up to him, another where Poseidon doesn’t love her anymore by the time he finishes.

Answer 3

The Sea Protected and Nurtured Horses

(I realize this is a 5 year old question, but it was so interesting I had to look into it.)

Probably, Poseidon was linked to horses because the Sea was seen as protecting and nurturing horses. This belief would be truly ancient, pre-dating Mycenaean Greek culture by nearly two millennia.

Vedic Connection

Since Greek Mythology is a daughter of PIE Mythology, (and the ancient Greeks themselves had significant Steppe Ancestry) the first question is: do other Indo-European cultures link horses and the Sea?

And the answer there is a resounding: Yes!

WHAT time, first springing into life, thou neighedst, proceeding from the sea or upper waters, Limbs of the deer hadst thou, and eagle pinions. O Steed, thy birth is nigh and must be lauded.

Rig Veda CLXIII (emphasis mine)

To him let us address the song well-fashioned, forth from the heart. Shall he not understand it'

The friendly Son of Waters by the greatness of Godhead hath produced all things existing. [...]

Here was the horse's birth

Rig Veda XXXV (emphasis mine)

These two verses from the Rig Veda - the first in praise of the horse itself, and the second in praise of Agni, known as the Son of Waters - both state the the horse comes from the Waters, or that the Waters were responsible for the horses birth.

If there is a strong linkage between Horses and the Sea in both Rig Vedic culture and ancient Greek culture, the mostly likely explaination is that both cultures inherited it from their common ancestor, the PIE culture.

Why Does the Sea Protect and Nurture?

Which brings us to the question of why the PIE people associated the horse and the sea. In The Horse, the Wheel, and Language David Anthony describes how the steppe economy would force Yamnaya / PIE herders to winter in the marshlands where major rivers emptied into the Black Sea, Caspian Sea, and Aral Sea, since the steppe was too cold and dry to support their herds during the harshest months of the year.

Since horses preferentially foal in the spring, PIE people would have spent all winter near the Sea, and the major rivers (Dnieper, Don, Volga, etc.) watching their mares become more and more visibly pregnant.

Then in the spring they would go out into the steppe and foal.

It would have been natural, then, to see the horse as "proceeding from the Waters" - since the Sea and the Rivers seemed to have protected and nurtured the mares and their foals through the hard, cold winter months.