Squirrel Girl is a very popular Marvel comics character, but I never heard about any live action adaptation in any development. She was considered a mutant too but then came out as not being one. So I wanted to know who really has the filming rights for Squirrel Girl, is it Fox or Disney? Or any other studio? Has it ever been discussed and cleared officially?
Freeform have announced a Marvel TV Show, New Warriors, and have included Squirrel Girl in the line up. She has been cast and will be played by Milana Vayntrub.
Excerpt from link:
It’s unclear if New Warriors will take place in the Marvel Cinematic Universe, as it will be Marvel’s first comedy TV series since it began producing its own content in Hollywood. The tone of the series will probably determine whether the show can work in the MCU. The Marvel films have a lot of comedy, but they never become outright farcical. If New Warriors goes for that kind of comedy then it may not fit in with the larger MCU
Based on the above, I'd have to say the rights must belong to Marvel/Disney.
Full details can be found here