Gulliver makes the mistake of hiring pirates to man his ship on his fourth voyage, and once at sea they mutiny and leave him on an island, the land of the Houyhnhnms. It is here that Gulliver encounters the Yahoos. Gulliver takes the Yahoos to be some type of animal and offers a description.
"Their heads and breasts were covered with thick hair... but the rest of the bodies were bare... They had no tails and often stood on their hind feet..."
Gulliver also describes them as deformed and highly disagreeable. The Yahoos are noisy, smelly, dirty, and unbathed. They eat almost anything, but love eating garbage.
Upon meeting Gulliver, the Yahoos climb into the trees and begin defecating on him. The attack comes to an end when one of the Houyhnhnms arrives. His time with the horses is spent trying to attain the level of perfection they embody, while the horses themselves are interested in how Gulliver could be so much like the Yahoos yet still a rational being.