Sorry for bumping, but I found something.
There's this movie called "Don't Look Under the Bed", that stars a girl who has help from an imaginary friend (appears as an african american teenage boy) to defeat this guy who is the Boogieman. The imaginary in question is named Larry, and he's pretty funny, and kid-like. No, even more. Toony, and unpredictable, as the girl (named Frances, and has red hair, LOL, like Frankie) seems to realize about him. But sadly, he's slowly turning into a boogie as the movie progresses, well, as much as I gather.
Spoiler Below Frances: You ARE turning into a boogieman, aren't you?
Larry: *trying to hide the changes by attempting to get under Frances' bed* What makes you say that?
"If a person stops believing an imaginary friend too soon, he/she might turn into a boogieman."
LOL
[EDIT]I'm watching it now. SAVE DARWIN, GUYS!
There's the info:
"Movie. Erin Chambers, Eric "TY" Hodges II, Jake Sakson. (1999) A logical girl must accept help from an imaginary friend to defeat a boogieman that is framing her for community pranks."
[EDIT EDIT] Ah, finally, something Wikipedia is right about - the movie's full info, if you don't know about this movie:
http://en.wikipedia.org/wiki/Don't_Look_Under_the_Bed