Does it have to be the end of modern Equestria? I'm writing a story that explores the apocalyptic frozen north, where the old pony kingdoms met their end to the windigos. It's an apocalypse(technically a cataclysm), so it counts, right?

It counts yes, absolutly, Apocalypse means basicly the end of the world, Doesnt need to be modern, it can be future, or past, It needs to be either Apocalyptic, or post apocatlypic, So your story is Fine Ima gonna read it, sound interesting.