Though to an outside observer, they are by all rights very much human-looking, they do indeed behave as animals. They are filthy and matted with dirt, they stink, and while they are capable of an omnivorous diet, they seem to prefer meat and garbage. Specifically, they are noted to consume nearly everything that is prohibited by the biblical and Levitical food codes. Of the Yahoo, Gulliver states they are: "the most filthy, noisome, and deformed animals which nature ever produced . . . " and they are "restive and indocible, mischievous and malicious." The Yahoos are not simply feral and animalistic, they are man-animals who are naturally vicious and represent the crudest and most corrupt of mankind's primal nature. Swift describes them in deliberately offensive and off-putting terms, frequently drawing them into metaphors with dung. On the evolutionary scale, the words Swift uses to describe the Yahoos are "degenerating by degrees."
Gulliver is painted as being midway between the two opposite ends of this spectrum, both figuratively and literally. He shares some similarities with both races, in some ways like the hyper-rational, and innocent Houyhnhnms, and in other ways like the filthy, emotion-driven Yahoos. At first, Gulliver refuses to admit having anything at all in common with the Yahoos, or vice-versa. He is shown to detest them, and is horrified upon further examination by the Yahoos' similarity to him. Yet he still lacks the humility to see himself as any sort of Yahoo, instead his pride leads him to the absurdity of trying to become a horse. Gulliver will endeavor with admirable determination to improve upon himself; he will strive to change himself into a more horse-like state, but he will obviously fail. To his dismay, he is quite clearly more of a Yahoo than a Houyhnhnm.