In Gulliver's Travels, how do the Houyhnhnms act as satire?
The Houyhnhnms, a race of intelligent horses, act as a representation of a purely logical society. When Gulliver is first learning their language, he talks with his master, and one of the first things Gulliver speaks of is how horses are treated in his own society:
My master, after some expressions of great indignation, wondered "how we dared to venture upon a Houyhnhnm's back; for he was sure, that the weakest servant in his house would be able to shake off the strongest Yahoo; or by lying down and rolling on his back, squeeze the brute to death."
(Swift, Gulliver's Travels, eNotes eText)
The Houyhnhnm can't understand how Yahoos (humans) can be the dominant species, since they are so much weaker than horses. The irony here is that while humans have domesticated horses, they use them for war and labor, and have not attained the levels of peace that the Houyhnhnms have. Meanwhile, the Houyhnhnms think of Yahoos as little more than animals, because they are so weak and have no ability to learn. The Houyhnhnms also live by mutual agreement with each other, instead of by battle; the Yahoos fight over food and valueless rocks, and show greed and distrust instead of reason.