Mega Swampert, Mega Sceptile, and Mega Diancie all confirmed.
Mega Sceptile is Grass/Dragon with the ability Lightning Rod. That is awesome. Thank you, GameFreak. Although Lightning Rod is a weird ability for a grass or dragon type. But, it might give him a free switch-in if you predict your opponents moves properly.
Mega Swampert's ability is Swift Swim, which doubles his speed in rain.