Despite all the rumors and finger pointing, it's learned that Fox Studios and Marvel Studios apparently get along, or at least have a good enough relationship to swap characters.
Deadpool screenwriters Paul Wernick and Rhett Reese revealed the deal to Playlist.
"We were looking down the list of the 400 [‘X-Men’] characters that Fox owns, and [Negasonic Teenage Warhead] jumped out and we were like, ‘yes,’ ” Wernick said. ” ‘I don’t care what her powers are. She’s gonna be in the movie.’ ”
It's explained that Wernick and Reese changed the powers of Negasonic Teenage Warhead in the Deadpool movie which required the okay from Marvel Studios, with Marvel in turn requesting Ego The Living Planet.
“Kurt Russell [‘s Ego the Living Planet] in the new Guardians movie was the character that Fox swapped with Marvel to [change] Negasonic Teenage Warhead powers,” offered Wernick.
This isn't the first time mention of a character swap between Fox and Marvel has become known, as according to reports, Marvel wanted to trade the rights to Daredevil for Silver Surfer and Galactus. Fox obviously said no, and Daredevil's rights went back to Marvel and became the Netflix TV series (we can further guess Marvel Studios never wanted DD for movies as he's now on TV, which is separate from Marvel Studios; so it's safe to assume Marvel Studios did want Surfer and Galactus for a film as Marvel requested the characters from Fox).
Regarding the negative tension between Fox and Marvel, it's been said to be due to Marvel CEO Ike Perlmutter's issues with Fox owning the film rights to certain Marvel characters. Reportedly, Perlmutter has gone so far as to order Fox-owned Marvel characters killed off in the Marvel Comics and replaced on Marvel merchandise so as not to promote the Fox Marvel movies.
The good news is that Marvel Studios president Kevin Feige had Perlmutter and his Comics and TV people removed from being involved with Marvel Studios and the Marvel movies, so we see that Feige has a good relationship with Fox Studios. More evidence of their good relationship comes from the fact that Feige worked with X-Men producer Lauren Shuler Donner who offered a while back she is open to an Avengers and X-Men crossover movie.
The Deadpool movie also happened to include a Kevin Feige Easter Egg.