I may be speaking out of context here so please forgive me in advance, but from what I understand with comics is the character is licensed to the publisher so they can easily allow other artists and writers to create their own versions of the character and stories.
The Big2 (Marvel and DC) own all the characters that they publish, regardless of who created them or who has done stories with them. They can basically do whatever they want. Other publishers, like Image, focus on creator-owned work and all rights belong to the creators.
Now with the movie rights, Marvel owns the characters outright and license some of them to Fox (stupid move, but that was before Marvel Studios and Disney started their thing), like FF, X-MEN and Spider-Man. Seeing that Deadpool classifies as an X-Man, Fox has rights to make a movie about him. They have creative freedom with the character, but so long as it adheres to the... Um... 'character' of the character.
Interesting other bit of info on this: Marvel is trying to reduce the attention and exposure of characters that are licensed to Fox, in an effort to make them worth less money. They can then buy them back, like they did with Daredevil. Fantastic Four have no presence in the comics at the moment and don't feature in any Marvel promotional material any more. Then there is Wolverine
who is going to die in the coming months and stay away for an indefinite time
Getting rid of free marketing for Fox means less income for their movies, making it cheaper to buy the licenses back. Just a little extra info that I thought might be relevant in this thread.
EDIT: Shit sorry guys, didn't mean to type an essay.