Here is my backstory for Fenrir if he existed in the Pony POV Series and not the Upheaval Series:
Fenrir is the personification of ravenous hunger, desire, and the worst parts of winter. Nobody knows who exactly created him, but he was around before the universe was created. When he first met Luna he fell madly in love with her and told her he wanted to make her his bride. When he refused, he tried to take her by force using an army of wolflike monsters called the wolven. He was beaten though, and he and his army were banished into a lifeless rock in the physical world. Unfortunately, this lifeless rock would eventually become the planet Equestria is on. He would spend most of the events of the story asleep and regaining his strength because the fight severely weakened him. When he finally awoke, it was around the time Luna first became Nightmare Moon. He tried to take her by force once again but Celestia and Luna were able to beat him and his wolven back, Luna taking his eye in the process. He retreated once again and is now currently biding his time for another chance to strike, his wolven testing Equestria's defenses from time to time with various attacks.
So... What do you guys think?