Why does Gulliver have such a strong reaction against the Yahoos when he first sees them?

Gulliver walks inland until he comes across a field of strange creatures. After observing them for some time he comments, "Upon the whole, I never beheld in all my Travels so disagreeable an Animal, nor one against which I naturally conceived so strong an Antipathy." Soon Gulliver comes to realize that these are actually naked human beings behaving like cattle. Gulliver comes face to face with one of them. He hits it with the side of his blade when it comes at him violently. The animal-like human (which Gulliver later learns is called a Yahoo) cries out, causing the rest of the forty Yahoos to surround Gulliver.