What did Gulliver see at the house of Houyhnhnms?

In Jonathan Swift's Gulliver's Travels, Gulliver's last voyage takes him to the land of the Houyhnhnms. There, he finds two kinds of creatures: A monkey/human hybrid of filthy and deceiving creatures called the Yahoos, and a species of talking horses called the Houyhnhnms.
Gulliver stays at the house of one of the Houyhnhnms and finds they are far superior to the people that Gulliver knew and dealt with in England. The Houyhnhnms are noble, honest, and peaceful creatures who care about others in addition to themselves. They don't even have a word for telling a lie. Instead, they can only refer to the act of lying as "saying a thing which is not."
Gulliver is so enamored with the Houyhnhnms and their way of life that he wants to stay there, forever, despite his family back in England. However, while the Houyhnhnm family that Gulliver is staying with wants to keep him, the rest of the Houyhnhnms are convinced that he is just a really smart Yahoo, and that he will be nothing but trouble in the future. They vote to banish Gulliver from their island after he has lived there for 5 years.
When Gulliver leaves, he is picked up by a passing boat, but Gulliver has gone mad: He associates every human being with the despicable Yahoos he met on the island of the Houyhnhnms, even his family.