Essentially internet horror stories or a myth passed around other sites, to frighten readers and viewers. The word, "Creepypasta" comes from the term, " copypasta ". An internet slang term for a block of text that gets copied and pasted from website to website. Creepypastas are sometimes are supplemented with pictures, audio or video footage related to the story. Typically with gory, distorted or, otherwise, shocking content.
Suicidemouse.avi was the grandfather of all lost episodes. It showed Mickey Mouse walking down a endless street with random piano keys playing in the background. The video gets more distorted and music turns static. See for yourself.
Also, Sonic.exe is a very well known Creepypasta. There is even a clone of the game to download. Although it's it well done, the clone will never have the same fear factor as the story did.
Lastly, Slender Man was introduced in 2009 in the Something Awful forums. Slender Man was not well know untill 2012 when Mark Hadley released, "Slender:The Eight Pages".