No one knows how the "ponies" got on the island, but one myth says that they are descendants of mustangs that swam there from a shipwrecked Spanish galleon (a large sailing ship). The most likely explanation is that they are the descendants of horses that were brought to the island in the late 17th century by owners on the U.S. mainland who wanted to avoid taxes and laws requiring them to be fenced. Although they are called "ponies," they are actually horses. Their diet of grasses and seaweed and the harsh environment prevent them from growing to full size, which makes them look more like ponies.