She has actually been in development for some time, starting as a sketch for a FIB pony, then being a finalized designed pony, and now as an Equestria Girl. Her clothes were going to be different to what they are now, with her originally wearing a skirt and stuff, but I couldn't find a design I like, so I settled on generic FBI/FIB clothes.
In the EQG universe, Fed doesn't go to Canterlot High or Crystal Prep, instead, like how that world's Twilight wants to go, she's part of the Independent Study Program. Where, in my headcanon, they wear uniforms dependent on what they're studying. In Federal's case, FIB clothes.
Now, I am expecting a little hate on this, with Federal being a Bureau horse and all, as well as the whole 'FIB' concept. But please, try to refrain from it. c: