User blog comment:Jacknapier10/Which characters cannot appear in the Marvel Cinematic Universe?/@comment-24901406-20140504155203/@comment-24901406-20140504214117

That's the good thing about the deal Fox don't own "mutants" they've just blocked Marvel from using the word so Pietro and Wanda can still be mutants, just referred to by a different name as the concept is still out there.

As for Deadpool it was simply because Marvel sold him off to someone else before Fox got a hold of him, but he is still separate for all legal purposes (especially reversion which is why Fox are rushing in that X-Force film).

Also the aliens can also be used by Fox, that's why I listed them (think they come under the Fantastic Four rights as well as the Avengers).