"Arthur, my mustache is touching my brain..."—The Tick (animation), That Mustache Feeling
Someone is about to turn into a monster. Or they have something inside them that is definitely not supposed to be there. Or they wake up to find that they are missing some bits. Or they learn, too late, that they are a character in an MPreg fanfiction...
Welcome to the lovely land of Body Horror. Simply put, this is any form of Horror that is based primarily on the body visibly mutating and developing in out-of-control, hideous ways. Instead of a clean, smooth, shiny (and often quick) change from one form to another, as with many Transformation Sequences, it's painful, Squickily organic, or just plain disturbing and played for all the Nausea Fuel it's worth.