If you think about it, all of the Equestrian Ponies have human counterparts in that world, even Twilight Sparkle does. And the Sunset Shimmer we know is a unicorn from Equestria. Could this be what her human counterpart looks like? Could she have returned to town without her Equestrian counterpart noticing, became worried and angry that Sunset had stolen her identity in her absense, and altered her hair to keep the people who were closest to her from confusing her with the fony?