Like Brobdingnag, Houyhnhnm (pronounced "whinim") Land is completely cut off from other nations – no one on Houyhnhnm Land has ever visited another country. This kind of isolation appears to be good for producing relatively virtuous societies. After all, the chief problem Gulliver sees with Lilliput and Laputa – their tendency to fight and conquer other peoples – isn't really possible on Brobdingnag and Houyhnhnm Land, where there are no other peoples readily available for conquest. At the same time, there are two distinct kinds of people living on Houyhnhnm Land, and it is the differences between these two that form the final part of the satire of Gulliver's Travels.
Gulliver arrives on Houyhnhnm Land by chance. After a really brief stay in England, Gulliver becomes captain of his own ship. He sails towards the South Seas when suddenly his men mutiny against him and lock him in his own cabin. Eventually, they maroon Gulliver on an island – Houyhnhnm Land.
When Gulliver first starts exploring this island, he runs across a herd of deformed animals with hair on their heads and covering their genitals but leaving the rest of their bodies bare. They seem agile, but they also tend to sit around on their butts a lot. The females have bare faces, without the long, goatish beards of the men, and their breasts ("dugs" (4.1.4)) hang down almost to the ground. These creatures are violent and easily frightened. When Gulliver strikes one with the flat part of his sword, a whole bunch of them swarm around him throwing feces, until he thinks he's going to be smothered in poo.
Just as Gulliver thinks he's going to suffocate in poo, another resident of the island comes to his rescue: a kind, gentle looking gray horse who seems to frighten these gross animals away. The horse seems fascinated by Gulliver, and particularly by Gulliver's clothing. As Gulliver hears this horse apparently speaking to another horse, he realizes that the horse's neighs and whinnies (from which the word "Houyhnhnm" comes) are slowly starting to make sense to him. The horse keeps saying the word "Yahoo" and gesturing to Gulliver.
The gray horse leads Gulliver through his own house and out to a kind of stable where a bunch of those vile beasts from the earlier scene are kept chained to a wall, surrounded by bits of raw donkey meat. Suddenly, Gulliver realizes the awful truth: these grotesque, violent, brutal, cowardly, hairy-but-also-way-too-naked creatures are, in fact, humans just like Gulliver. The horses, which are the reasonable creatures of this island, call humans "Yahoos," and keep a tight leash on them, because otherwise they'll misbehave.
This slow introduction to the Yahoos (gross humans) and the Houyhnhnms (lovely, smart horses) makes humanity unfamiliar and horrible to the reader. Just as the tininess of Lilliput and Blefuscu make the problems and wars of Britain and France seem silly and insignificant, this moment of lack of recognition that Gulliver has with the Yahoos suddenly forces humankind itself to seem unfamiliar and revolting.
Houyhnhnm Land is the one place out of all the islands he visits where Gulliver wishes he could stay. Sadly, though, he is forced to leave: the Houyhnhnms have an island-wide assembly every four years where they discuss important matters. Gulliver happens to be the important matter at the current assembly. The Houyhnhnms all decide that, as a superior Yahoo, Gulliver might some day go off and convince all the other Yahoos to organize and rise up against the Houyhnhnms. They decide he's too dangerous to have around, so they boot him out of the country. Gulliver has to make his own boat and sail to a nearby island.
So, let's get a little more specific about the Houyhnhnms. We love them because, well, they're horses – who doesn't like talking horses? But Gulliver kind of worships them, and it's worth talking about why. Here are some of the characteristics Gulliver singles out for comment: there are no words in Houyhnhnm language for any of the bad things we humans do, including lying, power, greed, or jealousy. In fact, Gulliver has a lot of trouble explaining human nature to his best buddy, the Master Horse, because he keeps having to talk around things that the Master Horse has no concept of. The best example of this kind of talking around that Gulliver has to do is "the thing which is not" (4.5.6), a phrase that the Master Horse uses to get as close as he can to "lie" in Houyhnhnm language.
The Houyhnhnms don't need laws or a special class of lawyers because they are completely governed by reason. Breaking laws is not rational, so they don't need to spell out their codes of behavior. This is like a more perfect version of the less-than-twenty-words Brobdingnagian rule about law – the Houyhnhnms don't need to limit the length of their laws because they don't even need laws. They all agree about the rightness of what to do.
The fact that all the Houyhnhnms agree about law points to something else Gulliver loves about them: they don't understand opinions or factions. To have an opinion about something, you have to speculate about something you can't know for sure. The Houyhnhnms accept hard facts; anything outside of fact, you can't argue about, because by definition you can't know what the correct answer is. It makes no sense to argue about something you can never answer correctly. This is why the Houyhnhnms have no law.
As you may have guessed from the fact that the Houyhnhnms don't have arguments or differences of opinion, they are equally friendly with all members of their tribe. They value "friendship and benevolence" (4.8.10) above everything else. In fact, this friendship thing is so important to Houyhnhnms that they treat all of their children as their own, and will educate all the kids in the same way.
This total lack of preference for one Houyhnhnm over another means that they always, always arrange marriages for their children. There's no such thing as a love match. Families will get together and decide: oh, your daughter is smart? My son is attractive. Let's breed them together to get smart, attractive children. And Houyhnhnm couples never cheat on each other because it makes no sense – they're in this relationship for the kids, not for love or sex or anything. And it gets even more technical: Houyhnhnm couples are limited to one boy and one girl foal (a baby horse). If one couple has two girls and another couples has two boys, they trade one of their kids. If a couple is unfortunate enough to lose a child to an accident, they can have one more child to supply the loss.
This type of rigidity in family arrangements is hugely different from what we've seen in, say, Laputa, where the wives are constantly on the lookout for other men. Or even Lilliput, where infidelity is clearly enough of an issue that Flimnap suspects his wife of sleeping with Gulliver. The Houyhnhnms have managed to subordinate their feelings to logic – like horse-shaped Vulcans or something.
But, much as we love Spock, we have to admit that this particular aspect of Houyhnhnm society creeps us out a little, because it gets Gulliver into kind of a tricky moral area. To be clear, this is what Shmoop is saying, and not so much Swift. The Houyhnhnms have a strong class system. Gulliver refers to "the race of inferior Houyhnhnms" (4.8.11) who are born to be servants. These animals are allowed to have more kids so they can serve the high-born Houyhnhnms better. Different colors of Houyhnhnms also appear to be better suited to different stations in life (4.6.17). All of this stuff about selective breeding and maintaining racial characteristics is really common when you're talking about animals, which of course, the Houyhnhnms are. But they also think and speak, which makes us feel like there's a moral gray area in this portion of Gulliver's description of the Houyhnhnms. Thoughts?
One last point about the Houyhnhnms as a group: they do not get sick. Gulliver describes most human illness as the result of overindulgence: too much food, drinking, and luxury in general. Add to that doctors who mostly make everything worse, and you have a recipe for a lot of human suffering. The Houyhnhnms eat a restricted, balanced diet which keeps them healthy until they are ready to die of old age. They feel this readiness to die about ten days before they do so, which gives them time to say goodbye to everyone and then go off by themselves to pass away in privacy and dignity. Compare this death to the unnatural long life of the struldbrugs, and we think you'll get a sense of what Gulliver thinks about the horrors of old age – death seems preferable.