I can't help but notice that on Deerling's Serebii page there are updated formes for Deerling. So this got me wondering how they actually did it - it still says on that page that the method is unclear, so unless they hack (which is proved to be possible, apparently) I'm thinking they may have brought it from Gen V to Gen VI via PokeTransfer. Can someone with PokeBank test this for me?
My brother owns PokeBank but he'll probably ask me why I'm sending over a stupid little deer into my game. :P