Mystique is a fictional character appearing in American comic books published by Marvel Comics, commonly in association with the X-Men. Created by artist David Cockrum and writer Chris Claremont, she first appeared in the comic book Ms. Marvel #16.
This is going off the debut of comic book characters. In the cases of characters who were introduced in other media originally and added to the comics canon later along (ex: Harley Quinn) it'll go by actual debut year.