The three rumored pokemon were supposedly leaked earlier yesterday. And I thought spriting with each release had been done for Gen. VI! At least, I wasn't prepared for leaks like this. Now smealum says there are unveiled mega-evolutions, too!
I've now updated the sprites with backs (Thanks, smealum!), and I made Volcanion almost 20% bigger (from about 80 to 96 pixels). The sprite is a tad too big now, although it now sizes nicely with Gigalith and Heatran instead of being dwarfed by them. We have back references of Hoopa and Volcanion, so I adjusted my sprites accordingly. Volcanion's back was a lot more simple than what I had predicted. :/ Also, Hoopa's ghostly tail could look better.
Edit (6/8/2014): Added a preliminary version of Mega Diancie. The back sprite is, again, tentative.
These designs likely belong to Game Freak; I do not own them.