Quoted By:
Mega Rapidash
Fire/Flying
HP: 65
ATK: 130
DEF: 70
SPA: 130
SPD: 80
SPE: 125
Versatile: When M-Rapidash attacks it uses either ATK or SPA for calculating the move's damage, depending on which defensive stat is weaker on the target pokemon.
OR
Blazing Temper: When M-Rapidash is hurt by an attack it is weak to it gains +2 ATK and +2 SPA.
Yep, weak to Stealth Rocks, I know. It wouldn't get to OU I think, but taking a Sneaky Pebbles to the face if Blazing Temper is the ability makes it very strong. I'd include some Flying moves in the moveset to get STAB as well, attribute them to the incredible swiftness of Rapidash or something. The idea is that people wouldn't be sure if you're running a Versatile mixed attacker that can always hit your weakest stat, or one that will pump up into a monster if it gets hit and survives.