I immediately thought of this creepypasta when I heard "tulpa", and I'm pretty sure it's one of the more well-known creepypastas. Not famous or anything, but definitely not obscure.
Since I couldn't really figure out a way to post the story, here's the top hit I found when Googling "Tulpa Creepypasta". I've listened to this reading before, so I know it's accurate.

I used this week as an excuse to finally get around to reading this creepypasta. I actually hadn't read it before hand, and now I see why so many people like it! CREEPYPASTA- ►http://creepypasta.wikia.com/wiki/Tulpa PATREON- ►http://www.patreon.com/CreepsMcPasta SIDE CHANNEL- ►"CreepsMcPasta Plays": http://bit.ly/SideChannel ►"Game Reviews": https://www.youtube.com/user/FullPackageReviews SPECIAL THANKS- ►MrCreepyPasta: http://bit.ly/MrCreepyPasta ►ProfessorCreep: http://bit.ly/ProfessorCreep ►Michael Berkefeld: http://bit.ly/MichaelBerkefeld ►G.