Sunset Shimmer, to the best of my understanding, is a pony who was sent to Canterlot High by Princess Celestia. We've seen plenty of human counterparts including counterparts for all of the main characters and several minor characters. Does Sunset Shimmer have a known human equivalent in the My Little Pony canon?
It was recently revealed that she was indeed planned to have an human counterpart, but the episode that should have included that was cut because... reasons.
The only remaining proof of this is a tweet by Nick Confalone, former staff member for the Equestria Girl show. In the tweet he posted this claim:
My Little Pony: Equestria Girls
Season 2 Finale Thoughts
Nick Confalone - 10/15/17
The Mane 7 have come to expect that Equestrian magic will find them wherever they go from school halls to camp bonfires, cruise ships to amusement parks. Their friendship seems to be the sweet, lavender-infused honey that attracts the gnarliest of nefarious flies, buzzing with other-worldly enchantments. Like most heroes with geode superpowers, the girls have accepted their roles as always friends and often heroes, but the question remains: why does this keep happening? Many shows ignore this question completely to the detriment of their storytelling universe's reality-think: Murder She Wrote's Jessica Fletcher discovering hundreds of dead bodies, literally a new one everywhere she goes—but we're going to address this question head-on... because the answer is an essential piece of Sunset Shimmer's ongoing redemption arc, harkening back to the genesis of the Equestria Girls franchise.
Ever since Sunset's turn at the end of that first movie, her friends have reassured her that her past is in the past—what matters is the good she chooses to do in the PRESENT.
...But they're wrong.
Unbeknownst to Sunset Shimmer, the powerful ancient-Equestrian magic spell she cast to open the portal to another world in the first place created an imbalance that has not been resolved. Her arrival in the human world displaced an innocent character whom we've never yet seen, but whose absence can be felt every time Sunset appears on-screen. In this final special of season 2, we're going to reveal what happened to the human Sunset Shimmer.
As you can see, not only Sunset was meant to have a counterpart, but that counterpart was also meant to be going thru some troubles because of that. We can only assume this was indeed a reference to the mirror balance mentioned in the IDW comics story arc "Reflections" (the one that introduced the mirrors in the first place)... In that case the reflection were opposite versions of the original and meddling between the two dimensions caused the copies to progressively become more and more evil. I doubt EG would have followed the same concept, but it is probable that human Sunset lost something because of pony Sunset presence.
What would have actually happened? Did human Sunset went to a different school because pony Sunset filled her place? And how was the problem meant to be solved: two Sunset claiming they were twins (Two Sunset for the price of one... great!)? Did we miss our chance to get the pony Sunset back in Equestria and in the regular show???? Sadly, we will never know.