A tulpa is a thought form which, once someone keeps thinking about it and exerts their will to imagine it in more and more detail, somehow takes life and becomes a physical reality. A tulpa will behave as though it is the real entity it is based on, if any; it can walk, talk, interact with physical objects and so on.
However being a soulless construct somewhat similar to a golem, ultimately all tulpas begin to malfunction or become violent and malevolent.
The Ubermenscher from the X Files episode "Arcadia" is a tulpa. In "Home Again" the Band-Aid Nose Man and Painter is also one. Curiously, in Home Again Mulder claims "tulpa" is a theosophist mistranslation of "tulku" and that it has no basis in Tibetan Buddhism, whereas in Arcadia he identified the Ubermenscher as being "a tulpa, a Tibetan thought form". The tulpa is in fact a very old concept in Tibetan Buddhism, and entirely separate from tulku.