User blog comment:Jacknapier10/Which characters cannot appear in the Marvel Cinematic Universe?/@comment-24901406-20140504155203/@comment-24901406-20140511164223

Indeed, they can only use Spider-Man franchise stuff, but if a mutant fell under the franchise, or if they simply wanted to use the word (in the sense that anyone in the real world would use it) there is nothing actually stopping them, whereas with Marvel Studios there is a legal barrier preventing them from ever using the word.

Skrulls are accessible to Marvel (they confirmed that at Comic Con in 2010 after people were wondering why if that's why they had "Chitauri" instead, but let's face it: the contracts are very stringent, there wouldn't be a glaringly obvious loophole like that). Kree can also be used by Fox (admittedly this came from the age old "anonymous source" so take it with a pinch of salt, but nobody has confirmed or denied).

Sony have definitely sold back all their television rights including live-action (freeing up the Kingpin for the Daredevil TV show). The Fantastic Four are unclear, but Marvel have confirmed the X-Men are definitely off limits (but even if they did have it, they wouldn't be allowed to use the word "mutant" due to the Fox ban).