In the novel Gulliver's Travels (Jonathan Swift, 1726) it is the name of creatures with the appearance of horses, but endowed with reason and without any vice. This contrasts with other characters who inhabit the same island, the yahoos, human-looking but wild-behaving, selfish, envious and aggressive, whom the Houyhnhnm treat like cattle. During his time in this society, Lemuel Gulliver ended up admiring the wisdom and nature of horses, and despising his peers whom he compared to the degenerate race of yahoo. The word houyhnhnm means "horse", and by its etymology: "perfection of nature"; all this according to Swift, of course.
