To be fair, there are so many mutants in the X-Men universe that it’s hard to compile a list of this nature. I know someone is going to mention something crazy like Gringleberg Jones that was seen in a single panel in a 1981 comic, that was only released to a single comic shop.
Pretty sure I just made that up, but you never know with so many mutants out there. Also, if Marvel wants to discuss Gringleberg Jones, I’m ready to sit down and write it.