The Horners are called that because they each have one horn, about six inches long, in the middle of their foreheads. They have light brown skin and three-colored hair, with red at the bottom, then yellow and green. They dress in white robes, and wear no shoes. Radium mining is their chief industry, and they decorate the insides of their houses with the radioactive metal. (At the time when the book was written, radium was considered a miraculous substance, and its hazardous qualities were not yet known. Perhaps, being Ozites and all, the Horners are immune to radiation poisoning, but I have to wonder if their odd appearances are due in part to the heavy radioactivity of the area.) While the radium makes their homes quite pretty on the inside, the outsides are simply made of dull gray stone. The Horners argue that they actually LIVE indoors, and that the Hopper houses are actually quite ugly inside. The Horners are known for their terrible jokes, at which they all laugh uproariously. Oddly enough, as raucous as the adult Horners are, they raise their children (or their daughters, at least; we don't see any sons in the story) in a quite puritanical fashion, forcing them to be quiet and polite. During the time of Patchwork Girl, the Horners are led by a chief named Jak. (Get it?)
It was actually one of the Horners' bad jokes that led to war at the time Ojo and Scraps visited the cavern. Diksey Horner joked that the Hoppers had less understanding than the Horners, because they had fewer legs. His thinking was that the legs are UNDER a person and used for STANDING, but not surprisingly, the Hoppers didn't get it. Diksey eventually explained the joke and conflict was averted, but it wasn't the last time that trouble would arise between the two groups. In Royal Book, the Hoppers and Horners are at war once again, and it takes Glinda's magic to bring them into line. Hopper-Horner conflict has since become sort of a running gag in Oz stories, with mentions of wars between them appearing in Forbidden Fountain and Blue Witch, among others. They're pretty much always offstage, though, so we don't know the details.
Fred Otto reused the Horners in his short story "Mombi's Purple Polkadot Vest," in which it's revealed that Mombi's four-horned cow (mentioned in Land) is actually a four-horned Horner named Phogg under enchantment. Because of his extra horns, Phogg was a wizard, and served as town leader. At the end of the story, he is planning to take back his old position, so that raises the question as to what would happen with Chief Jak.