Watch A Video Timeline Of Every Single 'X-Men' Comic Character
The X-Men of Marvel Comics are famous for being so numerous, and so interconnected in plot and history, that unraveling the connections between all of them is a herculean task. For example, take this fairly comprehensive X-Men timeline video, which runs eleven minutes and has to sprint in order to fit every team member into that running time. Along with the basic character bios, you'll get a detailed breakdown of decades of X-Men history, including many of the later revisions made to allow older stories to make sense in context with later tales.
Note that these are all proper X-Men members. Characters who were part of related teams such as the New Mutants won't show up unless they became proper X-Men members. These skipped characters, such as Franklin Richards, might be a prominent mutant characters, or could even have strong associations with the X-Men. But the skipped characters weren't actually members of any of the real X-Men teams.
Will this help you guess who might show up in X-Men: Apocalypse? Maybe. For that, focus on the first half of the 1980s in this timeline, though we could see characters later introduced (in real world time) show up as well. The films aren't very strictly related to the real-world or in-story continuity of the comics.
Note: there are major spoilers here for various X-Men stories, including character fates and identities that have been retconned and reimagined at various times over the years. There's also a good account here of the basic swings in popularity enjoyed (or suffered) by the various comics series, all of which help influence story development and the introduction of many characters.
[via IGN]