Hydreigon was actually an interesting card... for autumn Battle Roads, that is. Back when Eels/Rayeels varients and Garchomp/Altaria were being played during the first week of battle roads, I saw quite a few Darkrai/Hydreigon lists playing a one-of of this Hydreigon to snipe and get 2HKOs on Swablus and Tynamos.
However, in the current format, you would have to be an idiot in order to play this Hydreigon. It's weak to Landorus, and Terrakion OHKOs it, both of which are some of the most popular cards in the game. Overall, I'd give this card a 3/10. While it isn't playable in the current metagame (nor will it ever be playable), we can be thankful that at least it's not one of those boring filler rares that TPCI likes to curse us with.