If Discord = Crowley...that would default Celestia to being Bobby, though because the two XD I'm having a hard time visualizing Celestia saying Bobby's catch line without being unable to take it seriously. XD
Now, if Discord were Lucifer, going after AppleBloom, being one of the three that unwittingly provided some disharmony and let him break out, makes a bit more sense. Then again, why would we want to make sense?
Even though I didn't realize that was supposed to be Apple Bloom, it's not bad. Would have liked to have seen a more dynamic pose, like loading a gun, with the demon knife, or something. Since the roles are based off of age it's predefined who is who, but I feel you could have used more allusion in the clothing. Finally, I can't help but think there has got to be a joke that could be made with the Colt. P.S. interested who you'd pick for Castiel and Bobby.
Yeah, I could have come up with something more dinaymic, but this was really just a speed paint that took me less than an hour to make, so I didn't had enoiugh time to come up with cool poses. I don't know really know much about Supernatural, only had seen about 1 season of the show, so I can't be sure of which pony would be those characters you mention.