SURPRISE! Watch our old pal Hydreigon emerge from the shadows and be the only dragon that counters them with his dark type. GF must have had this planned when they made him. :p
Not really paying attention were you? The official art, not to mention one part in this official video disproves that idea of yours.
Looks like Fairy is at least neutral towards Dark. Even though we don't know how effective Dark will be against Fairy, Hydreigon is going down just as fast as any other dragon when facing a Fairy type.
I gave up and did the 3DS transfer anyway... 3DS XL FC: 2492-4470-1418 / White 2 FC: 0047-4149-5185 / White FC: 0348-0626-6780