Knowing mutants - in order of appearance in the X-Men tittles
I'd considered only those who was born with the X-Gene. For example, Mimic, Juggernaut and Sauron aren't in my list.
X-Men (1963), Uncanny X-Men (1981), New Mutants (1983), Alpha Flight (1983), X-Factor (1986), X-Terminators (1988), Excalibur (1988), Wolverine (1988), X-Force (1991), X-Men (1991), Cable (1993), X-Men Unlimited (1993) ...