I believe you are thinking of a tulpa, which is a Tibetan term. A tulpa is not the same as the Western idea of a thoughtform though. A tulpa is given life through a certain procedure and takes on a physical form that can be seen by anyone, as real as another human being. One of the best descriptions of it, along with an idea of what can go wrong, is in the book Magic and Mystery in Tibet by Alexandra David-Neel. Another good book dealing with a tulpa is An Unlikely Prophet by Alvin Schwartz.