I don’t think I’ve written yet about Ozian people made of mud. According to some religions, humans were originally formed from mud, and we still contain a lot of the same elements, but obviously there’s more to our bodies than that. The people of the China Country would have been made from clay. And there’s a mention in Grampa in Oz that the wizard Gorba tried to turn the Princess of Perhaps City into mud so no one else would marry her, but she becomes a flower fairy instead, credited to how “sweet, lovely and good” she is. This is something that comes up from time to time in Ruth Plumly Thompson’s books, as with how Mombi could make Orin into an old woman with magical powers, but not a hag. But the first really direct reference to mud men appears in Royal Book, when the Scarecrow is sliding down the beanpole. He runs into a group of people called the Middlings, whose bodies and clothes are formed from mud, with dried grass as hair, roots for arms and legs, golden teeth, and coal buttons. The mud is constantly shifting, which means their features often change and sometimes have to be patted back into place. Their eyes glow “like small electric lights,” but they still need to use a lantern.
As with a lot of the odd tribes of Oz, we don’t learn that much about these beings, but it appears that they spend most of their time digging with shovels and picks. We don’t find out what they’re digging for, although maybe it’s something to replenish their bodies. The only Middling named is Muddle, who works for the King. The monarch’s preferred means of address seems to be “Your Mudjesty,” although the Scarecrow calls him “Your Royal Middleness.” He demands a toll from the Scarecrow and considers keeping him even after that, but Muddle drops him after deeming him useless. We’re also told the words to the Middling National Air, which includes the line, “Oh, what is so lovely as mud!” I’ve seen several people point out the similarity to Flanders and Swann’s “Hippopotamus Song” (“Mud, mud, glorious mud”), although that came later and probably wasn’t directly influenced by it.
Ploppa, the giant turtle in Yellow Knight, also waxes poetic about the “delicious squg and glug” of mud.
Both of Lin Carter’s Oz books have fairly similar encounters with mud-men. In Tired Tailor, the wizard Wudj sends Dorothy, Pastoria, Snip, Pigasus, and the flying horse Skyhi underground to a place called Undertown, located in a cavern and inhabited by people made of mud with pointed heads, tree roots for hair, and glowing coal for eyes. They ride on giant earthworms, and their town is surrounded by lakes of molten magma and pools of mud.
Fire-flies, glow-worms, and fire flowers provide light, and the inhabitants apparently eat mud pies and dirt stew and drink root beer. Their ruler is Duke Down, assisted by the Grand Mudlump Unda. There are some indications that Undertown is part of a place called Down, which could be linked with the layer of the Underworld mentioned in Yellow Knight and the place visited in Hungry Tiger. That raises the question as to whether King Dad rules only Down Town itself, or all of Down, in which case the Duke might be a vassal of his. The muddy place in Merry Mountaineer is above ground in the Quadling Country. Called Mudville, presumably a reference to Casey at the Bat, it’s made up of mud huts. It’s unclear whether the inhabitants are made of mud or just covered in it, but we do know they try to coat visitors in mud, which is likely why they don’t get many. The ruler is Queen Muddalinda, whose officials include the Lord High Muckedy-Muck Muddle (must be a common name for mud-men) and the muscular Big Dipper.