Why do the Houyhnhnms initially consider Gulliver similar to a Yahoo?

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Illustration of Lemuel Gulliver lying on the ground, tied down by tiny Lilliputians, with crowds and small buildings in the background, from Jonathan Swift's Gulliver's Travels. — Gulliver’s Travels Lemuel Gulliver in Lilliput, illustration from an edition of Jonathan Swift's *Gulliver's Travels*.

In Jonathan Swift’s Gulliver’s Travels, the Houyhnhnms initially mistake Lemuel Gulliver for a Yahoo because of his physical appearance.12

The Yahoos are a race of brutish, filthy, and greedy humanoids that inhabit the same land as the rational, horse-like Houyhnhnms. Because Gulliver possesses the form and physical characteristics of a human, the Houyhnhnms categorize him as a member of this degenerate species. Although Gulliver is initially civilized and attempts to distinguish himself from the creatures, the Houyhnhnms remain curious about his nature, eventually concluding after hearing about his country and history that humans are no more reasonable than the Yahoos themselves.1234