This contract thing is confusing, does Fox own every mutant character that there exists? Can Marvel use any mutant as long as they appeared out-side X-men comics at some point? Of course they can't say that they are mutants or reference the X-Men. My guess is that Fox owns everything X related, but have exclusive rights to some characters that can't be used by Marvel, even if they appeared in books outside X-men. Guys like Wolverine who has appeared in other books outside X-Men could be an example of this. Perhaps Cable isn't exclusively linked to Fox, like Quicksilver, Wanda, or Viper but both companies can use them. It's just a matter of fitting the characters into their universe. Marvel doesn't have much a reason to include Cable into their universe right now, but Fox does.