Someone is about to turn into an alien, or has something growing inside them that is definitely not supposed to be there. Sometimes similar to The Puppet Masters, but in that case, the suspense comes from the implanted alien slowly taking over.
Sometimes, this implies getting your Face Full Of Alien Wing Wong. Alternatively, this is any form of Horror that is based primarily on the body developing in out-of-control, hideous ways. Parasites are usually involved.
Made most famous by the Alien quadrilogy, but done before that in the Doctor Who story "The Ark in Space".
Naturally, there's quite a bit of crossover with both Painful Transformation, Emergency Transformation, Came Back Wrong, and Transformation Trauma. See also Your Head Asplode, Squick, and The Virus.
For a character or Mook who has this as their backstory, see Was Once A Man and/or Tragic Monster.