Yahoos are the human-like creatures that Gulliver first encounters in the Country of the Houyhnhnms. Not recognizing their link with humanity, Gulliver describes the Yahoos as animals: " . . . deformed . . . . Their heads and breasts were covered with thick hair . . . but the rest of their bodies were bare . . . . They had no tails and often stood on their hind feet . . . ." He concludes with, "I never beheld in all my travels so disagreeable an animal."
Although they are human in form and feature, the Yahoos are, indeed, animals. They are filthy and they stink. They are omnivorous but seem to prefer meat and garbage. (Significantly, they eat nearly everything prohibited by the biblical and Levitical food codes.) They are "the most filthy, noisome, and deformed animals which nature ever produced . . . " and they are "restive and indocible, mischievous and malicious."
The Yahoos, however, are not merely animals; they are animals who are naturally vicious and represent Mankind depraved. Swift describes them in deliberately filthy and disgusting terms, often using metaphors drawn from dung. In terms of their evolution, the words used to describe the Yahoos are "degenerating by degrees."
Swift positions Gulliver midway — figuratively and literally — between the super-rational, innocent horses (the Houyhnhnms) and the filthy, depraved Yahoos. Gulliver, however, reacts to the Yahoos with immediate and overpowering detestation and is horrified by the Yahoos' similarity to him. He lacks the humility to see himself as a sort of Yahoo. Rather, his pride leads him to try to become a horse. Gulliver will try with admirable determination to improve himself; he will try to change himself into a more horse-like state, but he will fail. He is, simply, more of a Yahoo than a Houyhnhnm.