"Oh, for the love of... Tia!" The call echoed through the halls of Canterlot castle.
"Coming, Lulu!" Princess Celestia deposited a small filly into a playpen with a few other young ponies, then took off through the halls at a gallop. It didn't take long to find her sister and co-ruler, as the sounds of dishes being shattered and shelves toppling made a handy beacon for her. When she arrived at the Royal Dining Hall, she was torn between frustration and amusement at what she saw there.
The floor was littered with broken dishes and the remnants of a lunch she rather doubted anypony had gotten to partake of before disaster struck. Disaster, in this case, came in the form of a very young colt. Like the others, he was too young to have his own cutie mark yet. Like the others, that didn't appear to have stopped him. Gracing his withers was an image of a cartoon bag of bits, complete with the stylized B that was used to represent the currency.
"I've found another one," the Princess of the Night said needlessly. She was clearly very frustrated. Celestia suspected that the fact that the colt in question had somehow made his way up to the rafters -- the trail of broken dishes and toppled decorations indicated the path he had taken up a curio cabinet and across a series of mantlepieces -- and was taunting Princess Luna mercilessly was only partly the source of the frustration. The fact that, at some point in the chase, he had apparently flung most (if not all) of the contents of the dessert tray at her was another part. Lastly, the colt was the sixth such mysterious appearance that day.
"So I see," Celestia said, stifling a chuckle.
"Not one word," Luna warned.
"Of course not!"
"Have you had any luck determining the source of these... incursions?"
Celestia shook her head. "None yet."
"At this point," Luna said, glaring up at the colt, "I suspect we shall need a miracle to deal with this."
Celestia grinned. "I disagree," she said. "I think what we need is a foalsitter."
"Princess Cadance is busy with her responsibilities in the Crystal Empire," Luna objected.
Celestia nodded. "She is," she agreed, "but that's not who I had in mind." She turned her attention to the colt in the rafters. "Would you mind coming down here, please?"
He stuck his tongue out at her. "You're not the boss of me!"
Luna was aghast. "YOUNG STALLION!" she yelled in the Royal Canterlot Voice. "YOU GET DOWN HERE RIGHT THIS INSTANT!"
"Yes'm," he said meekly, then gingergly made his way to the edge of the rafter before sitting down and beginning to bawl.
Luna sighed. "What is it now?"
"It's too high!" wailed the colt.
The Princess of the Night rolled her eyes. "Whoever you have in mind, I would greatly appreciate it if you would contact them immediately," she said, before flying up to the rafters to give the young colt a ride down.
"Princess Celestia!" Twilight Sparkle, newly made Alicorn Princess of Friendship, galloped up to her mentor and nuzzled her familiarly.
"Twilight Sparkle," Celestia responded. Twilight winced internally when she realized that her mentor hadn't called her "my most faithful student", although she reminded herself that the reason for this was that she had, for all intents and purposes, graduated. "I'm so glad you and your friends could make it."
"What's the crisis?" Rainbow Dash said, darting back and forth in the air, shadow-boxing imaginary villains. "Whatever it is, we are on the case!"
"I am most gratified to hear that, Rainbow Dash," Princess Luna said as she alighted next to her sister. "However, I am uncertain whether fisticuffs shall be the answer to the problem."
"That's not what you were saying a few minutes ago, Lulu," Celestia said sotto vocce.
"I was... hasty."
"So, what is it? Diamond Dogs? Timberwolves? Discord acting up again?" Rainbow Dash grinned confidently.
"The problem is," said Princess Celestia, "ponies."
"Ponies?" Twilight Sparkle and her friends were caught off-guard.
"In particular," Luna said, "a small group of young colts and fillies."
Twilight frowned for a moment before suddenly getting an idea. "You want us to teach them about the Magic of Friendship?"
"Well," said Princess Celestia, "yes and no." She began to pace back and forth, trying to figure out how to explain the situation. "See, the problem is that these particular colts and fillies have no place they belong."
"Ohhh," said Pinkie Pie sadly, "they're orphans?"
"Not exactly," Princess Luna put in. "In a sense, yes. Their parents, or, rather, creators, are not here in Equestria."
"Creators?" Twilight tilted her head in curiosity.
"These foals are, for want of a better term, imaginary."
Rainbow Dash burst out laughing. "Good one, Princess! Imaginary foals!"
"I can assure you that this is no laughing matter, Rainbow Dash," Princess Luna said. "These... 'imaginary' ponies have done very real and substantial damage to Canterlot castle in just half a day!"
"So... what can we do to help?"
"I've come to the conclusion," the solar Princess said, "that these foals need supervision. More than that, however, what they need is a playmate. To them, the simplest of tasks can become an enormous adventure. Frankly, if it weren't for the... collateral damage..." Princess Luna winced at this term, shaking out her mane briefly, and Celestia shrugged her wings. "I would gladly entertain them here in Canterlot. As it stands, however, I can think of only one pony with the energy to keep up with them and the imagination to meet them halfway between our reality and the imaginary ones they seem to reside in."
Twilight Sparkle frowned, looking around at her friends. "Princess Celestia," she said hesitantly, "surely you don't mean--"
"Oh, but she does," said Princess Luna. "Pinkie Pie, would you do us the honor of taking charge of these... delightful young... whimsical..." She trailed off with a look of distaste.
Pinkie Pie gasped. In an eyeblink, she was directly in front of Princess Luna, a look of unparalleled excitement on her muzzle. "You mean," she said, "I get an extra-special, super-duper important mission from the Princesses just for me?"
Princess Celestia covered her muzzle with a forehoof to hide her grin. "Yes, Pinkie," she said. "An extra-special, super-duper-ultra-important mission from us. Just for you."
Pinkie began to tremble with excitement. Twilight and her friends took a cautious step back. As the three princesses and four other ponies looked on, Pinkie rocketed into the air, leaving a trail of streamers and confetti in her wake.
"WHOOPEEEEEEEEEEEEEE!"
Princess Luna shook her head in amazement. "You are correct, sister," she said. "I can think of nopony better suited to deal with these... imaginary ponies... than she."