The official Japanese Pokemon Twitter has revealed M Houndoom-EX from Red Flash. Thanks goes to Michael S. for the translation!
M Houndoom-EX – Fire – HP210
Mega Evolution – Evolves from Houndoom-EX
When 1 of your Pokemon becomes a Mega Evolution, your turn ends.
[R][R] Inferno Fang: 80+ damage. You may discard all [R] Energy attached to this Pokemon. If you do, this attack does 80 more damage.
When a Pokemon-EX has been Knocked Out, your opponent takes 2 Prize cards.
Weakness: Water (x2)