One of these, of course, was Fox, who ended up nabbing the cinematic rights to both the X-Men and Fantastic Four.
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.
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.
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.