I love The Appendix to the Handbook of the Marvel Universe website, so I thought I'd consult their listing of obscure mutants to find some of the most interesting forgotten mutants who almost certainly lost their powers:
Aardwolf: A crimelord who looks like an aardwolf, for no particular reason
Adahm: A telekinetic French midget who robs banks
Amber: A lizardy college student who can shapeshift, but only in her sleep
Anti-Matter: A skeleton in a space suit who looks like a Scooby-Doo villain
And that's just the letter A!