The Slender Man Mythos is the collective body of myths, stories, creepypasta, videos, and blogs focusing on Slender Man. The original mythos was started by Victor Surge in June 2009 on the Something Awful Forums, and have since exploded and spread through the internet and sub-popular culture, especially since the creation of Marble Hornets.
The main 5 Slender Man video series include Marble Hornets, TribeTwelve, EverymanHYBRID, DarkHarvest00, and MLAndersen0. MarbleHornets, TribeTwelve, and EverymanHYBRID are currently the top 3.
Because no particularly specific canon still exists that defines the specifics of Slender Man, the mythos has evolved in a generally similar manner across separate Alternate Reality Games to include all the basics of Slender Man.
Slender Man is also part of The Fear Mythos along with other entities such as The Rake.