User blog comment:Jacknapier10/Which characters cannot appear in the Marvel Cinematic Universe?/@comment-5350299-20140428113836/@comment-24892167-20140502111313

I personally doubt that most of the villains are specifically owned by a certain studio. Exceptions are those who are clearly tied to a certain hero, like Red Skull to Captain America, Magneto to the X-Men or Green Goblin to Spider-Man.

But take Viper for instance, Fox could use her, since the character is also related to Wolverine ([]).