On the summer solstice of the thousandth year after Nightmare Moon was banished to the moon, a mare of the Royal Guard, is patrolling the walls of Canterlot Castle. She hears a loud evil cackle from above her.
"Muahahaha, after a thousand years, my time for revenge has finally come!" It's Nightmare Moon! The legendary Mare in the Moon herself! The guardsmare readies her spear.
"You! Nightmare Moon! You won't get away with this!" she yells up to the villain. Not being a unicorn or pegasus, there is nothing she can do, however.
Nightmare Moon looks down at the fearless pony in surprise, then smiles and sticks her tongue out, blowing her a raspberry. Then she turns around and mockingly wiggles her rear at the powerless guardsmare. With a frustrated growl, the guard turns around and presents her own armored rear in defiance, smacking it with a hoof. Somewhere below the castle wall, there's a flash and the distinctive sound of a camera shutter. She looks down, unable to see anypony, and when she looks back up, Nightmare Moon is gone. Oh no...
The next morning, she collects her daily newspaper and lifts a hoof to her face as she sees her rump, cutie mark clearly visible, wagging up at Nightmare Moon. Then she reads the headline.
Nightmare Moon's back! Moons mare knight. Knight mare moons back.