"I'm amused by how many people are claiming that Slender Man exists, or has existed for "centuries," when the first released image was in 2009, and then suddenly we see a surplus of imagery that supposedly came before 2009. More research into the origins of these images and how altered they are is required, as well as real research into the history of the artistic pieces that supposedly depict Slender Man in Egypt and Germany. I haven't seen any "source" material from Egypt, so here's me tackling Germany:

1. Der Ritter
As far as Der Ritter goes, let's start with historical context. It was incredibly common for themes of death and demons to appear in German wood carvings during that time period. Examine the Northern Renaissance as a whole, for instance; these themes appeared because of religious attitudes of the era. If people were constantly reminded of death and the afterlife, they would make better efforts to behave in morally appropriate ways. Skeletal figures, with or without faces, were incredibly common ways to achieve this. For instance, Hans Holbein the Younger's "Dance of Death" series from the 1530s features skeletal figures - some with indistinct faces - amusingly putting individuals of all social standing into mortal danger, reminding them that it doesn't matter who you are: You will die someday, so you'd better live a pious life.
"Der Ritter" by Hans Freckenberg, is a hoax. Firstly, no such artist has ever existed, and no known artist has ever used the name. The image "Der Ritter," is a photoshopped image based on "Der Ritter" of Hans Holbein's "Dance of Death" (also called "Danse Macabre") pieces, as seen here: http://annachronique.tumblr.com/post/4174303533/my-ear-trumpet-der-ritter-the-knight-from-der"