The X-Men mythology has been going for over forty years now, and plenty of writers have tried to fill out the histories of the characters before they made their first appearances.  Some have no more than a handful of flashbacks.  Others are a tangled mess - most notably Wolverine and Cable, but Professor X, Cyclops, Gambit and Bishop also take a lot of unravelling. 

Trying to track their appearances just using the issue indexes wouldn't be convenient either for you or for me - so this section will carry chronological lists of pre-debut appearances, for those characters where the history has become complicated.

Angel
Banshee
Beast
Cyclops
Gambit
Havok
Iceman
Juggernaut
Magneto
Phoenix (Jean Grey)
Professor X
Storm