Gulliver is used to treating animals as animals; he has never encountered an animal with human-like intelligence. His visit to the land of the Houyhnhnms and Yahoos reverse that expectation, as Gulliver discovers that he identifies far more with the Houyhnhnms than he does with the Yahoos, despite their species difference. This puts Gulliver in the odd position of seeing his own people as more like animals, while the animals they domesticate contain great potential for intellect.
But I must freely confess, that the many virtues of those excellent quadrupeds, placed in opposite view to human corruptions, had so far opened my eyes and enlarged my understanding, that I began to view the actions and passions of man in a very different light, and to think the honour of my own kind not worth managing....
(Swift, Gulliver's Travels, gutenberg.org)
Gulliver's new views show the reader how strange domestication of animals might seem to a race that does not practice it. While Gulliver believes human treatment of horses to be moral and humane, the Houyhnhnms do not agree, and are appalled that the horses do not rise up with their great strength. It seems that domestication might, in this case, be more similar to slavery, as the horses have the potential to become intelligent but are kept as working animals by humans.