Mega Beedrill and Mega Pidgeot revealed, along with Mega Latias and Mega Latios. I thought it was a hoax when hackers found Mega Latias and Mega Latios and they looked exactly the same, but apparently that's really how they look.
I'm pretty psyched about Mega Beedrill and Mega Pidgeot, though; two Pokemon that actually really needed a boost. Mega Beedrill has Attack and Speed boosts (as you might expect) and its ability is Adaptability, which I guess is decent. Pin Missile is actually quite powerful now and Poison Jab is usable. Thinking about it, it's probably better than Skill Link would have been, although Beedrill might need a bigger boost, like Huge Power, to be worth it.
Mega Pidgeot has No Guard and seems like it'll have a SpAtk boost. As a special attacker with a no-miss Hurricane, it's finally differentiating itself from Fearow, Dodrio, Staraptor, Unfezant, etc.