Νομίζω ότι αναφέρεται στον Ποσειδώνα σε ορισμένες απεικονίσεις όπου οδηγεί το άρμα με λευκά άλογα στα κύματα...

In at least one Greek vase painting Poseidon is shown in a scene of the Gigantomachia riding on the back of a horse. So the imagery of Poseidon the horseman was not unknown...
http://www.mythography.com/forums/index.php?showtopic=1058&s...

JSTOR: The Utilisation of Old Epigraphic CopiesPoseidon as a horseman is unusual in Greek art, but the Anatolian god is usually a horseman, often carrying a battle-axe on his shoulder. XII. ...
links.jstor.org/sici?sici=0075-4269(1918)38%3C124%3ATUOOEC%3E2.0.CO%3B2-O
This horseman aspect of the sea-god is merely poetical. Do we not still speak of the sea's 'white horses'? The racing, crested waves are galloping, rearing steeds."

This explanation we might perhaps regard as valid did Poseidon appear only in poetry as horseman. But it is another matter when we find him so figured in early art. On a fragment of Corinthian pottery not later than the seventh century B.C., Poseidon is represented riding on a horse. In his right hand he holds his trident fishing-spear, an attribute surely not of much use to the horseman! Then again, when we come to the ritual of Poseidon, we find that horses were solemnly sacrificed to him. Every ninth year, in Illyria, a yoke of four horses was sunk in the waters. Again, Pausanias tells us that the Argives threw horses bitted and bridled into Dione in honour of Poseidon. There is here no question of the "white horses" of the sea, for Dione, we know, was a fresh-water spring.
http://www.sacred-texts.com/cla/mgr/mgr12.htm