A lot of fantasy authors rely on orcs and elves as their core non-humans, which honestly feels lazy. Humanoid OCs—I'm thinking merfolk with gills on their necks, fox-eared beastkin, or even human-looking constructs made of stone—force more interesting worldbuilding because they occupy that weird middle ground. They're close enough to us that readers project onto them easily, but their single divergent trait creates cascading societal implications.
Like, take a society of nocturnal humanoids with cat-like eyes. Suddenly you're building cities for low-light conditions, considering how trade happens in a day-walker's world, what myths they have about the sun. It's not just 'they live in caves.' It's asking how that biology shapes their art, their warfare, their concept of beauty. That's where the magic happens, when the OC isn't just a visual gimmick but a seed for cultural logic.
I've seen too many stories where the winged humanoid is just a human who can fly, with zero examination of how flight changes architecture or social hierarchy. The best ones make that trait central to their conflict, like a cliff-dwelling race facing deforestation. That kind of integration elevates the whole setting beyond a generic medieval backdrop.