Board Thread:General Discussion/@comment-4637931-20160824174412/@comment-24674051-20160826172253

Also, to add to the Frozen fitting in, Frozen was inspired by "The Snow Queen", with is a classic fairy tale (and one that was incluided in Once in conjunction with Frozen). as for Brave, it is a modern day fairy tale, but it has all of the elements of the classic fairy tales, which allowed it to be easily incorporated.

MLP has nothing that would lend itself to fitting into the Once universe.