I had not heard of Slenderman until this question. From Wikipedia I found this:
“Origin:
The Slender Man was created on June 10, 2009 on a thread in the Something Awful Internet forum. The thread was a photoshop contest in which users were challenged to edit everyday photographs to appear paranormal. A forum poster with the user name “Victor Surge” contributed two black and white images of groups of children, to which he added a tall, thin spectral figure wearing a black suit.[3][4] Although previous entries had consisted solely of photographs, Surge supplemented his submission with snatches of text—supposedly from witnesses—describing the abductions of the groups of children, and giving the character the name “The Slender Man”:
We learn something every day.