In the Star Wars: Paranormalities trilogy, this is a common side-effect of Forceless possession for the victims. Aside from gaining a dark appearance and developing glowing red, crystal ball-like eyes, they may also have other mutations depending on the species of the host and intent of the Forceless symbiote. Such mutations may include ExtraEyes(sometimes in places they shouldn't be), extra body parts, an extra layer of skin, or having their internal organs shifted around in a way that makes the host dependent on the Forceless for survival. Also, the host is consciously aware of the mutations, and they are usually painful.
The Kin in Hivefledverse are the ghosts of young trolls who died in the Grand Highblood's torture chamber. Of the two most prominently featured, Laneen had the flesh stripped off her arms and an extra pair of horns surgically installed to make her look more like the Condesce's Helmsman, then had her throat cut when she clawed out her own tongue to render herself boring. Her partner Sennir has half the flesh stripped off his head and neck and a gaping gutwound. Gamzee escaped alive, but now has a broken horn and a Glasgow Grin. Various other ghosts have appeared, including a twelve-year-old with a broken neck, a boy with crushed legs, a girl with a severed arm which floats beside her shoulder stump and another girl whose arm is missing completely, and a boy who died from drinking dirty water in a manner the authors tried to be subtle about (most of his guts exited his body in an insalubrious manner).
The "Human in Equestria" story You Do (Not) Belong has its protagonist begin growing a unicorn horn after a few days in Equestria. The scene where it begins ripping a hole through the skin of his forehead, which he is then forced to tear off, is Poltergeist-level disturbing, and it looks like it will only get worse from here.