Houyhnhnms are a race of intelligent horses. The Houyhnhnms live in Houyhnhnms Land and lived alongside the savage, human-like Yahoos. It is speculated that the Houyhnhnms are related to the exiled horses of Abdera. In 1711, Lemuel Gulliver encountered the Houyhnhnms when he was marooned in Houyhnhnms Land by a mutinous crew.
Source material
The Houyhnhnms appear in Jonathan Swift's Gulliver's Travels (1726).