Beyond the discovery that we humans are all Yahoos, the main point of interest in Gulliver's long descriptions of these people is the comparison between European and Houyhnhnm Land Yahoos. Gulliver tells the Master Horse about (a) lawyers, who make a practice of defending opinions that aren't their own for money (a.k.a. lying, according to Gulliver); (b) wars, generally over differences of opinion rather than anything substantial; (c) doctors, who will poison people for money and who encourage hypochondria among weak, idle people for cash; and (d) Ministers of State, whose main job it is to betray the previous Minister and advance his own ambition.
The Master Horse tells Gulliver a bunch of things his people have observed about Yahoos:
Despite the fact that European Yahoos look better than Houyhnhnm Land Yahoos, with their shaved faces, clipped hair, filed nails, and clothing, in essential nature they are the same. All Yahoos, no matter where they (or we?) are from love war, theft, sex, luxury, medicine, and lying.In fact, both the Master Horse and Gulliver decide that the Houyhnhnm Land Yahoos are better than European Yahoos because they don't try to hide their vicious natures under a mask of cleanliness and civilization. They are much stronger than Gulliver, and more able to stand the heat of the sun, thanks to their nakedness and outdoor living. To the Master Horse, it's far weirder to see a Yahoo pretending to use reason to explain things that cannot be rational, like war and lying and so on, than to see naked Yahoos acting according to their gross instincts.We do get an origin story for the Yahoos: they are probably the descendants of a shipwrecked couple who arrived on the shores of Houyhnhnm Land many years ago. By the time we reach the present day of Gulliver's Travels, they have completely lost all language and technology – again, yet more proof of the degeneration of mankind. (For more on this idea, check out our "Character Analysis" of the people of Glubbdubdrib.)There is a racialist edge to this comparison of European and Houyhnhnm Land Yahoos: the Master Horse calls Gulliver a "perfect Yahoo" who differs from other Yahoos in the "softness, whiteness, and smoothness of [his] skin" (4.3.9). And when Gulliver eventually gets banished from Houyhnhnm Land and is forced to find a nearby island, he encounters a group of local people naked, sitting around a fire, who shoot arrows at Gulliver, creating a clear parallel between the Houyhnhnm Land Yahoos and Pacific Islander populations in the South Seas.This idea that people living outside of Europe were somehow closer to nature or less tainted by civilization was a common one in Swift's day. Still, despite the fact that Gulliver's satire is largely directed at the European Yahoo, we can't ignore that he does seem to favor class and race distinctions. For more evidence of this kind of racialist thinking, check out the "Character Analyses" of the Houyhnhnms and their selective breeding and of Glubbdubdrib and Gulliver's analysis of the dwindling status of the English nobility.