What do these have to do with rationality?
Rationality includes instrumental rationality, and imaginary friends can be useful for e.g. people who are lonely.
deluding yourself into believing it has autonomy
Not sure of what exactly you mean by "autonomy" here, but there are plenty of processes going on in people's brains which are in some sense autonomous from one's conscious mind. Like the person-emulating circuitry that tulpas are likely born from: if I get a sudden feeling that my friend would disapprove of something I was doing, the process responsible for generating that feeling took autonomous action without me consciously prompting it. And I haven't noticed people suggesting that tulpas would necessarily need to be much more autonomous than that.
that if revealed to other people would make them concerned about your mental well-being
Someone might make his social circle concerned over his mental well-being if he revealed himself to be an atheist. Simply the fact that other people may be prejudiced against something is no strong reason for not doing said something, especially something that is trivial to hide. Also, the fact that tulpas are already a somewhat common mental quirk among a high-status subgroup (writers) can make it easier to calm people's concerns.
and imaginary friends can be useful for e.g. people who are lonely.
The instrumentally rational thing to do, when faced with loneliness, is to figure out how to be with real people. No evidence was presented in the original post that suggests that tulpas mitigate the very real risk factors associated with social isolation. Loneliness is actually a very serious problem, considering most of the research seems to indicate that the best way to be happy is to have meaningful social interactions. Proposing this as a viable alternative would require a very high amount of evidence. A post presenting that evidence would be something that belongs here.
There have been a number of discussions here on LessWrong about "tulpas", but it's been scattered about with no central thread for the discussion. So I thought I would put this up here, along with a centralized list of reliable information sources, just so we all stay on the same page.
Tulpas are deliberately created "imaginary friends" which in many ways resemble separate, autonomous minds. Often, the creation of a tulpa is coupled with deliberately induced visual, auditory, and/or tactile hallucinations of the being.
Previous discussions here on LessWrong: 1 2 3
Questions that have been raised:
1. How do tulpas work?
2. Are tulpas safe, from a mental health perspective?
3. Are tulpas conscious? (may be a hard question)
4. More generally, is making a tulpa a good idea? What are they useful for?
Pertinent Links and Publications
(I will try to keep this updated if/when further sources are found)
(Bear in mind while perusing these resources that if you have serious qualms about creating a tulpa, it might not be a good idea to read creation guides too carefully; making a tulpa is easy to do and, at least for me, was hard to resist. Proceed at your own risk.)
Footnotes
1. "Conjuring Up Our Own Gods", a 14 October 2013 New York Times Op-Ed
2. "Hearing the Voice of God" by Jill Wolfson in the July/August 2013 Stanford Alumni Magazine
3. "The Illusion of Independent Agency: Do Adult Fiction Writers Experience Their Characters as Having Minds of Their Own?"; Taylor, Hodges & Kohànyi in Imagination, Cognition and Personality; 2002/2003; 22, 4
4. Thanks to pure_awesome
5. "Sentient companions predicted and modeled into existence: explaining the tulpa phenomenon" by Kaj Sotala