Believe it or not, the word Houyhnhnms
appears in most English dictionaries. According to Webster's New World College Dictionary, 4th Edition
is pronounced either "hoo-IN-um" or "HWIN-um."
A Houyhnhnm, of course, is one member of a fictional race of intelligent and ethical horses that appears in Jonathan Swift's Gulliver's Travels. Their reliance on reason and their innocent ethicality is contrasted with the Yahoos, who Gulliver describes as "the most filthy, noisome, and deformed animals which nature ever produced." Because of his pride, however, Gulliver fails to see the similarities between the human race (including himself) and the Yahoos, whose "heads and breasts were covered with thick hair . . . but the rest of their bodies were bare. . . . They had no tails and often stood on their hind feet."
The Houyhnhnms and Yahoos represent two extremes of mankind, with Gulliver both literally and figuratively stuck between them: the Houyhnhnms represent absolute innocence and icy-cold reason, and the Yahoos represent the depravity of mankind.
As much as he tries to become like the horses, Gulliver cannot rid himself of his Yahoo-ness, and the Houyhnhnms send him away.