The reader has already seen Gulliver's pride operate to some extent in the earlier books. Gulliver identified himself with the giants in Book II, for example. Now he identifies himself with the horses. Gulliver's identification of himself with the giants produced only ludicrous results. But, in this book, his attempt to identify himself with the horses is more critical. The horses are alien to Gulliver; graphically, in their physical contrasts, they are not at all similar to him. Yet Gulliver thinks of the Yahoos as alien and animal. He makes traps of Yahoo hair. He makes shoes of Yahoo skin. He covers his boat with Yahoo skin and calicos it with Yahoo fat. Separating himself from his naturally depraved cousins, the Yahoos, Gulliver also separates himself from the European Yahoos. He is near to madness — because of pride. Swift warns us of this danger by using the phrase "devoted to destruction" when Gulliver is sent away by the Houyhnhnms. The phrase is theological, describing those with an excess of pride, who reject charity and humility.
When Gulliver says, "When I thought of my family, my friends, my countrymen or [the] human race in general, I considered them as they really were, Yahoos in shape and disposition, only a little more civilized . . . ," he is, in essence, rejecting the society (including wife and family) that has produced him. He seeks admittance into "the perfection of nature," the society of the Houyhnhnms. Nevertheless, even though Gulliver recognizes several Houyhnhnm maxims, including, "That nature is very easily satisfied" and "That necessity is the mother of invention," he does not recognize a third, implied maxim (a maxim understood by Houyhnhnms, but not by Gulliver): "Once a Yahoo, always a Yahoo."
