I came across this a long time ago when reading up on Tibetan Mysticism, and had need of the word a few weeks ago but couldn't remember or find it. But - no sooner do you put the question out there than the internet reflects the answer back to you...
Tulpas:
Any doubts I had were allayed by the fact they have their own website. And the "For Science" slogan in the upper left. Uh-huh. In any event you can't broach a topic like this without incurring a fair bit of "woo-woo", so you have to sift the "evidence" and "testimonials" with more than a grain of salt.
But seriously, this has some curious ideas embedded within it - as in, this voluntary creation of another personality in a way resembles multiple-personality disorder, only the one is the willful creation of someone the creator wishes (and presumably can get along with), the other is the involuntary creation of a personality that often isn't liked by the principal conscious inhabitant. And the idea of projecting it into the wider world around us, that it can assume it's own corporeal shape and grow independent of the creator, well, there's some serious food for thought.
Note: We do this otherwise, with people, children, technology, etc - but there is always the intermediary step of physical creation, with Tulpas there is not.
Related: Golem, Frankenstein's Monster, Shamanism, Spirit Guides, Psychopomps, Alexandra David-Neel oh, once you begin this one you can be led down a great many rabbit holes...