The Malvern Stables aren't just a place for Sean Kendrick to wallow in poverty and horse manure. They're also a key to the mythology of the island. When Sean describes that "The ceiling is held up with carved columns that depict wide-eyed men whose hands support the feet of other men in turn and again in turn, and at the top of all them are men with the heads of horses" (11.10), along with a painting of a man dragging a horse down into the sea, we get the feeling that the stables have been there a lot longer than Malvern has.
But what's with those columns? Do they mean that the horses are triumphing over man? Sitting at the top of the column could mean that they're superior beings to us puny humans. But they could also mean that they need humans to hold them up.