The proof is inevitable, slender beings are ancient.

With legends dateing back before the dark ages, with cave paintings and hyroglyphics as proof, slender beings have very obviously been around for a very long time. These lengends can be seen at the said link:

http://theslenderman.wikia.com/wiki/Slender_Man_in_Mythology_and_Culture

  There are hundreds of photo evidence and sightings, not only on slenderman, but on the rake, who is yet another slender being. Stories and photos of the Rake at the said link:

http://theslenderman.wikia.com/wiki/The_Rake