They are feral horses that were originally from stock that was either left there by shipwreck, or left when owners left the islands (depending on who you ask). They grow up on an island that isn't huge.. not a lot of nutrients. Lots of salt water and sea weed. Every year, they swim the horses from the outer island (Assateague, off the coast of VA I think) to the Chincoteague island. There the firemen auction the foals (and youngstock) to raise money for the island, and to thin the herd. Then the horses are swam back. Occasionally an Arabian stallion (or a few other breeds.. mostly arabian) are turned out to add outcross blood, usually for a few years until his oldest daughters are of breeding age (if they didn't sell), then he would be brought in, and another stallion (arabian or chinco) would be turned out.