This article is from the X-Men Comic Books FAQ, by Kate the Short (email@example.com) with numerous contributions by others.
Mimic, the original Phoenix, Longshot, and (possibly) the two Psylockes.
Mimic is Calvin Rankin, the son of a scientist, who got caught in an
explosion in his father's lab, and gained the ability to copy
superpowers of every superhuman near him, and keep all the powers until
the people he stole them from were over a mile away or so. He's been
killed a number of times in a variety of filler stories, and brought
back just as often to just die again. Scott Lobdell tried to retcon this
in a backup story somewhere by saying that Rankin was a latent mutant
who just got his powers started up by his father's explosion. While
there have been other latent mutants who have gained access to their
powers in such dramatic ways in Marvel history, Scott Lobdell is also
responsible for such continuity goofs as Storm declaring she resents and
hates her thieving heritage, and all of X-Men Unlimited #4, while the
Official Handbook of the Marvel Universe still has Rankin as a non-
mutant. This FAQ will go with the OHOTMU.
The original Phoenix (the one in UXM #101-137) was once Jean Grey, who
was a mutant, of course, but has since been retconned into being the
cosmic Phoenix Force itself, just pretending to be Jean Grey. As a
cosmic entity, Phoenix automatically is disqualified from being a
mutant; they have enough troubles as it is. See the entry on the Jean
Grey/Phoenix relationship question in this FAQ for more information.
Longshot was a genetically-designed being from the dimension of the
Spineless Ones. As an artificial life form, he cannot, by definition, be
a mutant; his "free will" could be described as a design malfunction,
but actually, it was programmed into him by Arize (Longshot #1-6). Even
if he showed up on a mutant detector, something for which there is no
textual support, he's still not a mutant, because nothing in his genetic
makeup happened by mutation. He was built from the ground up.
The original Psylocke, Betsy Braddock, is the sister of Brian Braddock,
aka Captain Britain. Both she and her brother gain their powers from
their not-entirely human heritage (their father was from Otherworld).
This was stated directly in her pre-X-Men appearances, as recounted now
in the "Captain Britain" trade paperback. However, there are statements
made in the X-titles clearly identifying her as a mutant--including a
caption where Betsy described herself as a mutant in UXM #213, and an
identification by the Master Mold in UXM #247. One reader points out
that telepathic powers are unusual even for the Otherworlders--Brian's
powers are more typical. While this doesn't prove that she is both a
hybrid and a mutant, it adds a little credibility to the notion.
Kwannon may have been a mutant, or it may have been simply genetic
engineering--it was revealed that she was a low-level empath, with her
source of powers undetermined. (However, this generally means "mutant
powers", especially where the X-titles are concerned.) The practical
upshot of all this is that since Psylocke finally occupied Kwannon's
body, the question of her mutancy no longer concerns her Otherworld