Book IV: While Gulliver is captain of a merchant ship bound for Barbados and the Leeward Islands, several of his crew become ill and die on the voyage. Gulliver hires several replacement sailors in Barbados. These replacements turn out to be pirates who convince the other crew members to mutiny. As a result, Gulliver is deposited on a "strand" (an island) to fend for himself. Almost immediately, he is discovered by a herd of ugly, despicable human-like creatures who are called, he later learns, Yahoos. They attack him by climbing trees and defecating on him. He is saved from this disgrace by the appearance of a horse, identified, he later learns, by the name Houyhnhnm. The grey horse (a Houyhnhnm) takes Gulliver to his home, where he is introduced to the grey's mare (wife), a colt and a foal (children), and a sorrel nag (the servant). Gulliver also sees that the Yahoos are kept in pens away from the house. It becomes immediately clear that, except for Gulliver's clothing, he and the Yahoos are the same animal. From this point on, Gulliver and his master (the grey) begin a series of discussions about the evolution of Yahoos, about topics, concepts, and behaviors related to the Yahoo society, which Gulliver represents, and about the society of the Houyhnhnms.
Despite his favored treatment in the grey steed's home, the kingdom's Assembly determines that Gulliver is a Yahoo and must either live with the uncivilized Yahoos or return to his own world. With great sadness, Gulliver takes his leave of the Houyhnhnms. He builds a canoe and sails to a nearby island where he is eventually found hiding by a crew from a Portuguese ship. The ship's captain returns Gulliver to Lisbon, where he lives in the captain's home. Gulliver is so repelled by the sight and smell of these "civilized Yahoos" that he can't stand to be around them. Eventually, however, Gulliver agrees to return to his family in England. Upon his arrival, he is repelled by his Yahoo family, so he buys two horses and spends most of his days caring for and conversing with the horses in the stable in order to be as far away from his Yahoo family as possible.
