The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Houyhnhnm
    n 1: one of a race of intelligent horses who ruled the Yahoos in
         a novel by Jonathan Swift