Source : Webster's Revised Unabridged Dictionary (1913)

Houyhnhnm \Hou*yhnhnm"\, n.
   One of the race of horses described by Swift in his imaginary
   travels of Lemuel Gulliver. The Houyhnhnms were endowed with
   reason and noble qualities; subject to them were Yahoos, a
   race of brutes having the form and all the worst vices of
   men.

Source : WordNet®

Houyhnhnm
     n : one of a race of intelligent horses who ruled the Yahoos in
         a novel by Jonathan Swift