Curse of the Werepony by dlazerous
Link - http://www.fimfiction.net/story/19077/curse-of-the-werepony

"An unfortunate turn of events finds Spike changed into an Earth pony.  Rarity unintentionally crushed his heart and now she has to fight hard to win back his trust, his friendship, and maybe something more."

Music used: Werewolves of London (1978) by Warren Zevon
I own nothing in this video.