Houyhnhnms are, of course, the noble horse-like creatures who reign over the humanoid occupants of their island, visited by Gulliver in Swift’s Gulliver’s travels. They are philosopher kings and have no word “to say the thing that is not true” – that is, they have no capacity to understand nor to express the concept of a lie. They are an equine parable of perfect pulchritude, even finding their inspiration for art in nature. The Yahoos are undisciplined, immoral, untruthful and even savage.
From Swift we also get Lilliputian, for the tiny citizens of Lilliput; and Brobingnagian, their colossal apposites.
Swift was a great defender of the language, and he left no explanation (beyond the obvious ear he had for imaginative invention) for his method of, or rationale for, word creation – except to express the fantastical and fabulous.