Before Marvel created its own independent movie studio in 2005, the company had for decades farmed out its various properties to several different entities throughout Hollywood. One of these, of course, was Fox, who ended up nabbing the cinematic rights to both the X-Men and Fantastic Four.
There are actually quite a few grey spots, as several different characters in Marvel Comics have had allegiances with multiple brands or individual franchises; Quicksilver and Scarlet Witch, for instance, were originally introduced within the X-family (as baddies), but eventually made their over to become major members of the Avengers and, later on, the Inhumans. This has resulted in both Marvel Studios and Fox sharing certain characters, which is why Quicksilver can be seen in both The Avengers: Age of Ultron and X-Men: Apocalypse.
Guardians of the Galaxy, Vol. 2 director James Gunn has recently popped up on Twitter (h/t Comic Book Resources) to talk about another co-shared property between Marvel Studios and 20th Century Fox: the long-lived, shapeshifting alien race known as the Skrulls.
Some specific Skrulls are at Fox. But the Skrulls as a whole are co-owned. https://t.co/2hJYkskLZ3
— James Gunn (@JamesGunn) July 6, 2016
While the co-sharing of these rights has been something that has been known for quite a while now, this reaffirmation comes at an interesting time: there has been lots of talk as of late that the two companies have begun to bury the hatchet (in 2014 Marvel disbanded the Fantastic Four in the comics, allegedly to tamper with Fox’s efforts at doing a rebooted film franchise, so the studios haven't necessarily been on the best terms in the past) and might even merge their cinematic holdings together, as has happened with Spider-Man in this summer’s Captain America: Civil War.
The possibilities of fully utilizing these extraterrestrial shapeshifters are nearly endless, especially considering that Marvel could retroactively change certain characters or their arcs with the revelation that audiences have been actually following along with their alien doppelgangers instead of the real individuals themselves (as famously happened in the 2008 Secret Invasion comic book miniseries), and this could even hypothetically be the basis for a potential cinematic rapprochement.
Then again, given the fact that the new X-Men television series (yet another indication of the growing cooperation between the two studios) aren’t within the X-movie canon, and that the inclusion of any Fox-owned characters probably wouldn’t occur until the MCU’s Phase 4 at the earliest, maybe fans shouldn’t get their hopes up for anything this elaborate just yet.
Captain America: Civil War is in theaters now. Doctor Strange opens November 4, 2016; Guardians of the Galaxy Vol. 2 – May 5, 2017; Spider-Man: Homecoming – July 7, 2017; Thor: Ragnarok – November 3, 2017; Black Panther – February 16, 2018; Avengers: Infinity War Part 1 – May 4, 2018; Ant-Man and the Wasp – July 6, 2018; Captain Marvel– March 8, 2019; Avengers: Infinity War Part 2– May 3, 2019; and as-yet untitled Marvel movies on July 12, 2019, and on May 1, July 10, and November 6 in 2020.