The local crime syndicate sees Miller's lack of ID as an advantage. Before too long, our poor former human is eyeball-deep in the scum of Canterlot. Forget going home - the challenge is trying to stay one step ahead of police, criminals, and a pair of persistent mares.
Human gets turned into pony. Then organized crime gets involved.
In the underworld of Equestria, one mysterious pony is fearfully whispered about. Only Vinyl and Octavia know why he left a trail of destruction through the criminal ranks. Now he's back, and, well...not a he anymore.