Definition of "Houyhnhnm"

I retain in my Mind the Lectures and Example of my noble Maſter and the other illuſtrious Houyhnhnms of whom I had ſo long the Honour to be an humble Hearer.

1726 October 28, [Jonathan Swift], “The Author’s Veracity. His Design in Publishing this Work. […]”, in Travels into Several Remote Nations of the World. […] [Gulliver’s Travels], volume II, London: […] Benj[amin] Motte, […], part IV (A Voyage to the Houyhnhnms), pages 340–341

"The Squire says you know a good deal about horses, so I suppose you understand the Houyhnhnm language? I'm learning it, and it is very nice," laughed Miss Celia, as Chevalita gave a little whinny and snuffled her nose into Ben's pocket.

1878, Louisa May Alcott, chapter 7, in Under the Lilacs

”The moment he called me his ‘gentle and reasonable Houyhnhnm,’” an archivist notes, “I should have known that the man was deranged.”

1995 February 19, Marilyn Stasio, “What to Read: Crime”, in New York Times‎, retrieved March 3, 2021

Certainly the philosopher was not a pessimistic luncheon companion. He […] loped off in the direction of the London Library, leaving me with an impression of twinkling eyes, laughter lines (he whinnies like a donnish Houyhnhnm), and a sensibility genuinely focused on what will happen in the next five minutes - or five years - albeit one steeped in knowledge of the past.

2014 January 28, Will Self, “John Gray: Forget everything you know”, in Independent‎, retrieved March 3, 2021