Medieval fanfic
This is awesome. The Got Medieval blog traces the history of fanfic all the way back to the middle ages, when enthusiastic Chaucer nuts wrote their own Chaucer sequels:

Chaucer seems to have attracted this sort of activity more than other writers--or possibly, we modern readers are more interested in tracking down this sort of thing when it's done to a writer we admire as much as Chaucer.

I would so write myself into Canterbury Tales. If I weren't reading all the slash that was surely around then...?

So, did they have fan fiction in the Middle Ages? The answer is "yes," though their tastes tended less towards slashfic ... and more toward self-insertion fanfic, the variety in which the author of the derivative work makes themselves a character in the original fictional world, usually a character who is so much better at everything...

So Mary Sues go back to to Middle Ages, although slash probably doesn't.