Here's a fun example, for anyone unaware; this tickled my mind – Lucifer.
In the beginning, we weren't sure that Venus was one thing, we saw it as (maybe) two, the Morning Star and the Evening Star. In Latin, one of those objects names was "Lucifer." It was literally just the name of a celestial object (Phosphoros in Greek, among a handful of other proper names).
There are a few ancient Hebrew (i.e. Canaanite) myths that associate the planet Venus with a god who attempted to usurp a supreme god's throne and became an underworld deity. As far as I knew, this isn't reflected in mainstream branches of Judaism; this is a relic of their days as polytheists.
Nonetheless, Latin Christians sure got a kick out of it, and incorporated that narrative into their mythos. To my knowledge, however, they did not make a association between "Lucifer" and "Satan". Those were two, separate mythical figures for hundreds of years of early Christianity.
I don't believe this association was popularized until Dante's fiction. Therein, the "Lucifer" we think of as the prideful pretty-boy who fell from heaven and took over a spicy underworld comes from medieval, Italian fiction. It has little to do with the myths of ancient Christianity.
I like to think of it like high school teachers. English teachers and History teachers both include Julius Caesar in their curriculum. English teachers (often, in my experience) base their understanding of Roman history on Shakespeare. History teachers base it off of Cicero, etc.
Unless you go to the source, it's some level of fan-fiction.
(Check me on some of those claims; I'm over-generalizing a bit, I know).