There were other mirrors, of course. Twilight found dozens locked away in Canterlot Castle, their strange magic glowing faintly in a dusty storage room.
This one had been a tiny hoof-mirror, made of pink plastic, encrusted with seashells by an excited filly centuries ago. “Daddy and me at the beach,” it said on the back, in faint scrawled hornwriting.
The world it led to was empty, just warm sunlight and the peaceful sound of waves. The sand castle was still there.