The distinction is that The Thing was unknown throughout the course of the movie - no one knew what “the thing” was bc it morphed and formed into humans and other creatures. Alien - the setting was strictly in outer space , we knew out the gate that it was an alien being. That movie didn’t take place on earth - when we think of horror we think, earth as the setting, and paranormal, psychological, evil beings manifested not of this Earthly world, but the setting is not in another planet or in outer space.