fan

Beginning in 1967, authors Lin Carter and L. Sprague deCamp wrote a lengthy series of novels based on Robert E. Howard's Conan the Barbarian character. They called this work "posthumous collaboration", claiming they were working from Howard's notes left behind at his death, but over time it became clear they were making up most of it themselves. They also made massive changes to Howard's already published Conan material.
Modern fan fiction probably originated with Star Trek fandom which in turn inherited many of its practices from science fiction fandom. The first known published Star Trek fanzine is Spockanalia, published in 1967. This community popularized many traditions from the science fiction community that are still in place today, including the concepts of crossovers, zine culture and public feedback. They also originated the idea of the Mary Sue or annoying wish-fantasy character who appears in some juvenile fan fiction.
Other fandoms were active in the same period as Star Trek, including The Lord of the Rings, The Prisoner, Doctor Who, and The Man from U.N.C.L.E. As time went on, the various fandoms began to take notice of one another, and by the mid-1970s a more widespread distribution of such fan fiction began. Much fan fiction was originally distributed as individual stories to friends and family. Some fandoms didn't publish their own fanzines until the mid-1970s. In the case of Man from U.N.C.L.E., this did not happen until 1976 with the publication of a Man from U.N.C.L.E. story in Warped Space.