Oh you guys. It's a fanbase like any other, just like what we have here.

This fan project in particular has quite a storied history as I understand it. I think it started out basically as a fan novel - just one of the most ambitious and complete works of fanfiction ever created. Apparently it was so good it spawned it's own fanbase and people started making derivative works of it. On the forum I'm on (yes, I like MLP dun dun duuun) there was a RP thread dedicated to the Fallout: Equestria universe.

I don't know the novel so I can't verify, but I assume this game is based off the same lore. So we have a giant derivative game based off a giant derivative novel based off a mash-up of a TV show and a video game. You could never do this back in the Commodore 64 days I tell ya...