A Mare Worth Fighting For is a side story written by Pacce. It is widely viewed as a "clopfic" and is meant for an age group over 18. It is the only side-story publicly accepted by Kkat to be canon with Fallout: Equestria.
You may find the story on its Equestria Daily post with the Main Story of Fallout: Equestria or here if you prefer. It is also available on fimfiction.net, here.