On the subject of mental NPCs it is realtivly common and I've heard the phenomenon is experienced by many writers as they try and write realistic characters they slowly lose control over the characters as stuff they planned no longer fits the character that they've made.

On Tulpas I have no experience with but Zigg has told us he was able to remove a malign Tulpa before and these two are based of Twilight and Pinkie Pie, some of the most benign characters in fiction. They seem to have a positive influence on Zigg and Zigg is comfortable with their presence. As things stand there is nothing to worry about. I'm currently debating if I should try and generate a Tulpa of my own though I strongly doubt my ability to do so. Interestingly enough I suspect my doubt will further make it less likely I will succeed and this realization about my doubt will further decrease my chances.