Tulpa

The tulpa is a Tibetan thoughtform. A thoughtform is a manifestation of mental energies. A tulpa is a being or object that is created through sheer force of will alone. The term stems from the works of French spiritualist Alexandra David-Néel. She claims to have produced a tulpa in the form of a monk that resembled the Friar Tuck character from Robin Hood legends. David-Néel states that the tulpa developed of life unto itself and she was forced to destroy it.

Since a tulpa is created through thought alone, as the creator's perceptions of the tulpa change, so to does the tulpa itself. The nature of the tulpa can only be affected by the mind, or minds, of its creator, or creators. For example, if one wanted a tulpa that had wings and could fly, the manifestation would appear thus. If the creator then decided the creature had too much freedom, they need only change their thoughts so that the tulpa no longer had the ability of flight. The wings would vanish, or at least become useless, and the tulpa would be grounded.

The tulpa, or a related apparition, has been used in works of fiction by numerous authors. An example of this is the creature from Stephen King's It. The creature is a thoughtform that gains powers and changes form based on the belief of the people living in the town. An episode of the science fiction show X-Files has a character who utilizes a tulpa to enforce the neighborhood rules and kills those that do not follow those rules.

Sources:
Tulpa; Wikipedia
It; Stephen King
The X-Files - Arcadia; Ep. 13 Season 6 1999