Very simply, the Houyhnhnms are horses; they are not humans. And this physical difference parallels the abstract difference. They are fully rational, innocent, and undepraved. Man is capable of reason, but never wholly or continuously, and he is — but never wholly or continuously — passionate, proud, and depraved.
In contrast to the Houyhnhnms, Swift presents their precise opposite: the Yahoos, creatures who exhibit the essence of sensual human sinfulness. The Yahoos are not merely animals; they are animals who are naturally vicious. Swift describes them in deliberately filthy and disgusting terms, often using metaphors drawn from dung. The Yahoos plainly represent Mankind depraved. Swift, in fact, describes the Yahoos in such disgusting terms that early critics assumed that he hated Man to the point of madness. Swift, however, takes his descriptions from the sermons and theological tracts of his predecessors and contemporaries. If Swift hated Man, one would also have to say that St. Francis and St. Augustine did, too. Swift's descriptions of depraved Man are, if anything, milder than they might be. One sermon writer described Man as a saccus stercorum, a sack filled with dung. The descriptions of the Yahoos do not document Swift's supposed misanthropy. Rather, the creatures exhibit physically the moral flaws and natural depravity that theologians say plague the offspring of Adam.
Midway between the poles of the Houyhnhnms and the Yahoos, Swift places Gulliver. Gulliver is an average man, except that he has become irrational in his regard for reason. Gulliver is so disgusted with the Yahoos and so admires the Houyhnhnms that he tries to become a horse.
This aspiration to become a horse exposes Gulliver's grave weakness. Gullible and proud, he becomes such a devotee of reason that he cannot accept his fellow humans who are less than totally reasonable. He cannot recognize virtue and charity when they exist. Captain Pedro de Mendez rescues Gulliver and takes him back to Europe, but Gulliver despises him because Mendez doesn't look like a horse. Likewise, when he reaches home, Gulliver hates his family because they look and smell like Yahoos. He is still capable of seeing objects and surfaces accurately, but he is incapable of grasping true depths of meaning.
