Poseidon has three main aspects: sea-god, earth-shaker, and giver of horses. As sea-god he could stir up the waves or calm them, while as the earth-shaker his power was terrifying. Only as the god of horses was Poseidon a clear friend to humans.

Father of Horses

The horse was an important part of his cult and myths, and among his titles were Hippios, of Horses, and Hippokourios, Horse Tender.

In some myths he is associated with horses from birth, in others he became a horse, and many different stories tell how he gave horses to humans, even crediting him with the bridle. (Also credited to his old antagonist, Athena, as Athena Hippia, although some myths made Poseidon her father in that aspect.)

Sophocles, in Oedipus at Colonus, has the Chorus praise Poseidon:

And I have more praise for this city our mother, the gift of a great divinity, a glory most great: the might of horses, the might of colts, and the might of the sea. For you, son of Cronus, lord Poseidon, have set her on the throne of this pride, by establishing first in our roads the bit that cures the rage of horses. And the shapely oar, well-fitted for the sea, in flying past the land leaps to follow the hundred-footed Nereids.
(710-5, trans. Jebb)

Two different myths made him the father of the first horse. One of these stories came from Thessaly, a centre for horse-breeding, and said that while the god slept, he emitted semen, which fertilized the ground and produced the first horse. (Burkert 1985: 138 and Hand/Rose: 102) An Athenian version says that he produced the first horse there; either by emission or by striking the earth with his trident.

He fathered two famous, immortal, horses: Pegasus and Arion. Pegasus was born by delayed action: Poseidon slept with Medusa, and later, when Perseus cut off her head, the horse Pegasus sprang from her neck. (There’s a lovely vase painting of this, with a tiny winged horse hovering above Medusa’s headless neck.)

Arion, along with the mysterious goddess Despoina, was born when Poseidon took stallion form to mate with Demeter Erinys. (He didn’t pick the easy ones, since Erinys means Fury, and in another version Arion’s mother was a Harpy.) This story, from Thelpousa in Arcadia, told how when Demeter was searching for Persephone, Poseidon stalked her. When she disguised herself as a mare among a herd of mares in the hope of throwing him off, he changed himself to a stallion and raped her.

Arion became a type of the swift, magical horse, who numbered Hercules among his owners.

Born a Horse?

The story of Poseidon’s birth is equally strange. He was the second son of the Titans Rhea and Cronos, a father who habitually swallowed his children because of a prophecy that one of them would overthrow him. According to standard myth, only Zeus escaped this fate, Rhea substituting stone for the infant, which Cronos duly swallowed. (Theogony II. 453-491)

Alternative versions, however, also allowed Poseidon to escape, while Rhea handed her husband a foal to swallow. Pausinias, in his Description of Greece (VIII:8:2), describes the spot where the infant was hidden among the lambs. (A peaceful start for such a violent, elemental god.) Diodorus Siculus (V:5) said that he was raised by the Telechines on Rhodes, magicians and metalworkers who in some versions were the children of either the Oceanids or Nemesis.

This tale of Poseidon’s birth, along with his connection to horses, has led some to say that the foal and the god were the same, although since Cronos swallowed it that would seem unlikely. It is interesting that Rhea did substitute a foal for her baby, however, given the close connection between the two.

Giver of Horses

As the god of horses, Poseidon also looked out for charioteers in the horse-races which were an important part of Greek life and sport. Twice he is mentioned in the Iliad as patron of charioteers, the first when Nestor’s son is about to race, since he taught the boy his skills, and the second time the young man has to swear by Poseidon that he did not cheat.

The god intervened in a chariot race involving his former favourite Pelops. The son of Tantalus, he was the god’s lover for a while, but later set himself to win Hippodamia (note horsy name). 18 other suitors had died after losing chariot races against her father, so Pelops asked the god for aid, and Poseidon gave him a chariot and winged horses.

He also gave a winged horse to Idas, and his sons Pelias and Nelius were suckled by mares. (Hard/Rose: 101)

Pindar, praising a successful charioteer, says:

Earth-shaking Poseidon, he is devoted to you, who rule over horse-races, and his thoughts are pleasing to you. His sweet temperament, when he associates with his drinking companions, surpasses even the bee’s intricate honeycomb.
(Pythian Odes VI:50, trans. Svarlien)

Further, at Onchestus, it appears that charioteers had their own rites at the grove of Poseidon:

And further still you went, O far-shooting Apollo, and came to Onchestus, Poseidon’s bright grove: there the new-broken colt distressed with drawing the trim chariot gets spirit again, and the skilled driver springs from his car and goes on his way. Then the horses for a while rattle the empty car, being rid of guidance; and if they break the chariot in the woody grove, men look after the horses, but tilt the chariot and leave it there; for this was the rite from the very first. And the drivers pray to the lord of the shrine; but the chariot falls to the lot of the god. (230-7)
(Homeric Hymn to Delian Apollo, trans. Evelyn-White)

Although most Graeco-Roman art shows him riding hippocamps, he did have a chariot with horses, at least in Homeric myth. The Iliad has one marvellous passage which shows him clearly as a powerful god (sitting on the mountain-tops like Zeus), then making the earth shake as he strides to the sea to mount his horses, who are not wetted by the water. (The Irish said the same about Manannan mac Lir, incidentally.)

There he sat, being come forth from the sea, and he had pity on the Achaeans that they were overcome by the Trojans, and against Zeus was he mightily wroth. Forthwith then he went down from the rugged mount, striding forth with swift footsteps, and the high mountains trembled and the woodland beneath the immortal feet of Poseidon as he went.  Thrice he strode in his course, and with the fourth stride he reached his goal, even Aegae, where was his famous palace builded in the depths of the mere, golden and gleaming, imperishable for ever. Thither came he, and let harness beneath his car his two bronze hooved horses, swift of flight, with flowing manes of gold; and with gold he clad himself about his body, and grasped the well-wrought whip of gold, and stepped upon his car, and set out to drive over the waves. Then gambolled the sea-beasts beneath him on every side from out the deeps, for well they knew their lord, and in gladness the sea parted before him; right swiftly sped they on, and the axle of bronze was not wetted beneath; and unto the ships of the Achaeans did the prancing steeds bear their lord.
(Iliad XIII: 15-31, trans. Murray)

---

References:

Burkert, Walter 1985: Greek Religion, Harvard University Press.
Hard, Robin 2004: The Routledge Handbook of Greek Mythology, Routledge (revised edition, original by H. J. Rose).
Puhvel, Jan 1997: Comparative Mythology, John Hopkins University Press.

Save