Just plucking at strings here but the film describes Zoroark as the "Ruler of Illisions". Well, Illusions are a lie, right. So Zoroark could possibly be Black's fronting Pokemon while the asofyet unannounced Pokemon fronting white's would be something along the lines of "Conveyor of Truth". For this thread, I'll call it "WhiteKing". It sounds cool.
While Zoroark uses its powers to shapeshift and produce illusions (going off movie only at this point), WhiteKing has powers that allow to detect the "truth" in situations and instantly know whether someone is lying to it or not.
How these would reflect into the game or not could possibly intern that "WhiteKing" cannot have its accuracy lowered, or be hit by attacks such as Future Sight, while Zoroark could automatically have a bonus in evasion, and/or have a moveset that revovles around abilities such as Transform & Substitute.
Just a theory I'm winging on here.