Its ability was already confirmed to be Clear Body a while back, though..
More Theorymonning time...
Mega Hydreigon
Dark/Dragon -> Dark/Dragon
92/105/90/125/90/98 -> 92/135/100/165/110/98
Levitate -> Tri-React*
Tri-React: Base Power of attacking moves is reduced to 40%, but hits three times
Something me, Xan, and Arpa came up with in Skype last night. Looking at its ability, it's basically the infamous Mega Kangaskhan's Parental Bond, but with an overall damage increase of 20% as opposed to 50%, with the ability to hit 3 times. Backed by a base 165 Sp.Atk stat, Mega Hydreigon remains as a proficient wallbreaker. The main draw, however, is that like Mega Kangaskhan's deadly combination of Parental Bond and Power-up Punch, Mega Hydreigon can make use of Tri-React and Charge Beam to gain as much as +3 towards its already high Sp.Atk, further bolstering its wallbreaking capabilities. Unlike Power-up Punch, however, Charge Beam does not have perfect accuracy, and the Sp.Atk boost only has a 70% chance of occuring rather than PuP's 100% boost rate. Factoring in accuracy, however, Mega Hydreigon's Charge Beam has a 87.57% chance of gaining at least 1 Sp.Atk boost, a 70.56% chance of gaining at least +2 Sp.Atk per use, and a 30.87% chance of gaining all three Sp.Atk boosts. A +2 Sp.Atk Mega Hydreigon is nothing to scoff at, easily taking care of many of OU's walls and defensive Pokemon, and +1 can pull its weight around as well:
Offensive Calcs
Unlike Mega Kangaskhan, Mega Hydreigon does not have access to any priority move, making it susceptible to being revenge killed by faster offensive threats, including Dragons, Fairies, and Fighting-types, and priority moves such as Ice Shard, Mach Punch, and Gale Wings Brave Bird, though it can comfortably take Sucker Punch. Though it may be able to 2HKO Unaware Clefable and OHKO Mega Mawile and Azumarill, Mega Hydreigon wants to avoid fairy-type attacks at all costs.
Defensive Calcs