The Houyhnhnms were just horses.
It was such a utopia that he became a brony centuries before Season One. Too bad the humans in that land were so barbaric.
- Perhaps ancient Equestria, to be exact. The Houyhnhnms were prehistoric Earth ponies long before they began to live alongside pegasi and unicorns, and interbreeding with them gave Earth ponies colorful fur and cutie marks centuries after Gulliver left. And because Humans Are the Real Monsters, the Yahoo were killed off.
- Perhaps the Yahoos fell victim to the Windigos and were frozen to death.
- Actually, in the book the horses decide to exterminate the Yahoos... by castration.
- It is also a rather efficient deconstruction before there was even a need for one: Houyhnhnm is clearly depicted as cold and alien, and Gulliver becomes a mad man estrangled from the rest of humanity.
Lilliput and Laputa have been wiped out.