This is the closest I could find:
https://comicvine.gamespot.com/profi...he-x-me/16483/
But the list only includes X-Men debuts, so even if for instance Franklin Richards appeared in 1968, he does not appear in X-Men before 1981 (as an adult).
Then there is Namor, which was never a mutant to begin with, but was retconned into one later.
Other examples of mutants that are not members of the X-Men are Mindworm in Spider-Man and Ursa Major, Darkstar and Vanguard in Hulk.
(Other than that, the list seems accurate, even if becomes a little problematic in Uncanny X-Men #168, where we have Madelyne Pryor, which is a clone. But that's a small problem compared to the next issue, where we are introduced to the Morlocks. They were so many and only exist in the background, and with a few exceptions, we never learn who they are, so they can obviously not be included.)