@Nathan - thanks for the elaboration on Bifnaz or whoever he was. I like our talks about the different patterns we find in story! Might have to do a larger post at some point about crossover characters and deconstruction crossover, fun stuff.

Fizban is a particularly annoying instance of this trope, showing up as Zanfib in the Deathgate Cycle by the same authors, whose explanations for why he isn’t actually the same character are tortured, at best.

A related trope is the borrowed character, taken from public domain works, or used without permission for fanfic. Characters like Dracula, Merlin, and Robin Hood tend to reappear in more mythic fiction, though often reinterpreted in some way. The deep end there is what TV Tropes calls the deconstruction crossover; probably the best known example is “The League of Extraordinary Gentlemen”, which includes Mina Harker from Dracula, Jeckel/Hyde, the Invisible Man, and more.