As Future Junior walked through Old Equestria alone, he was not lonely. What need had he for friends in an age without magic? The dragons refused to accept him because he was half unicorn, while the ponies saw the blood of their enemies flowing in him. Since neither side would accept him, he had no friends, choosing to focus on the ancient magic that his mother could wield.
But after the war, everypony pretty much kept to themselves to avoid betrayal. Dragons had always tended to work alone, but there were legends of one who had joined with a pony while the elements of harmony were still intact. This probably led directly to the war, but since Junior's parents were dead, he had no way of learning for sure. His father had died in the war and his mother died in his complicated childbirth.
Instead, he focused his attention on ancient lore. Which brought him here, to a place where ancient artifacts still might have existed. Specifically, his soul was looking for a time when he would be most happiest, when the two races were joined as one. There were legends of a mirror that could reflect time like a normal mirror could reflect light.
Future Junior found the mirror that could reflect time, and went to the ancient location of the withered Tree of Harmony. It seemed like a good idea to go back to a time when dragons and ponies got along, when the elements of harmony were still intact. The Mirror could take him there, inside of a box sealed by the remnants of the elements. So he went back to the past.
Before he went to present time, he stuck himself in a box that could only be opened with the elements of harmony so he would show up in the right time. Even so, he wound up coming out sooner than he had hoped (going backwards in time, this meant he wound up later than now).
Left in the box was only a mirror. The Mane Six finally managed to open the box, thanks to the lessons they learned through season 4. They all saw the Mirror inside. Clearly, the Mirror must be the key to their current crisis. (The current crisis was dealing with an evil version of Future Junior). They all fired their new Elements of Harmony from the Mirror of Harmony.
Magic did not work against the half dragon/ half unicorn that was their enemy. In fact, the mirror broke. "There is nothing your puny magic can do against modern defense."
"Oh no, the mirror was actually powerless," Princess Twilight Sparkle said.
"Wait," Twilight Sparkle said, "I have an idea. What if it wasn't the mirror, but what was reflected inside that was powerful?"
"What are you talking about? Obviously, that mirror had no power," Future Junior said.
"We all looked in that mirror, what did we see? We saw ourselves. Our friendship is the true power inside." And then they activated their true magic, which was their friendship reflected in the mirror.
The End