So, by far the most prevalent theory about the Slender Man's nature is that, at some point, be it after the original SomethingAwful thread, or long before, enough people thought of the Slender Man that He came into existence, bearing the properties we imbued Him with. Our being scared of Him, in short, made Him. Hell, perhaps he's some archetype of fear, dreamt up millenia ago, being thought and re-thought into existence throughout the ages, and Victor Surge's photos struck close enough to home to recreate him. The tupla theory explains a lot. The ability for Him to come to those who know about Him seems to gel with the idea that He was created by people having a defined enough idea of Him.
But it doesn't fit.
Firstly, we may be giving ourselves a little too much credit here, guys. It makes sense to some that Slender Man is made up of a number of primordial fears, and that when stuck together by Victor Surge, or whoever else back in history, they were strong enough to form Him. It makes much more sense to me - though this may be my perspective is coloured by Bleakley's theorizing - that those fears are there because humanity has had the Slender Man creeping around the edges all along, and evolution simply got rid of those who didn't know to run in the opposite direction. Tulpa theory points to us pre-dating him, but the ubiquity of entities throughout history sharing traits suggests that he goes back much further.
But secondly, this is the third of these theory sections, and the only thing we've found for sure is that no-one actually knows anything about Him. The Slender Man varies so much across different accounts that it is simply impossible that He was formed this way. You see, if thoughtforms or tulpas worked, they would be as a result of emergence - the process by which communication and feedback between small, simple agents, obeying simple, synchronised local rules, creates results beyond the capabilities of any one of those agents. Think termite nests. There simply was never sufficient communication and feedback, and never enough consistency. That variation in amost everything about him happened way too soon, and too widely, for any emergent result to occur. Simply put, the properties of the Slender Man are far too widely varying for them to also be solidified enough to have a real entity emerge from it, except for the barest agreed elements, and the remaining traits couldn't just be filled in by whoever was nearest. If the Slender Man was a tulpa, there would be one Slender Man, consistent in His manner. We have the opposite opposing us today.
I can't see the Slender Man being a tulpa. I mean, the ultimate proof would have been Robert's Core Theory working, kind of like how the A-Bomb proved the theory of special relativity - if it wasn't real, it wouldn't have worked. Except Core Theory DIDN'T work. I can see why it would be comforting to have Him be a creation of ours - if we brought Him into this world, it seems more achievable to take Him out of it again - but it simply doesn't fit.
The sickness hasn't gone away.