Foals can be cute, playful, and energetic.
They can also be annoying.
This was a lesson Nightmare Moon was figuring out all too quickly.
No sooner than she had returned to Equestria after a millennium of being banished to the moon, HER moon, than she found herself beset by an ingorant, innocent interloper. She had waited for so long to return, so long to seek revenge, and so long to take back the land that had scorned and shunned her unfairly for so long... she had waited and waited until the prison that hold her finally allowed her release... so long had she waited, oh so very long. She had returned to her former home, imprisoned the sister who had locked her away, made a grand, elegant appearance in front of her treacherous subjects, and locked the world in the perpetual grasp of night time...
And now this.
And so she sat, a flat, yet annoyed, expression on her face.
A foal sitting on her back.
She didn't even see where the little colt had come from. All that she knew was that suddenly, after teleporting back to the ruins of her castle home in the Everfree forest, after telling off that student her sister had taken under her wing, she stood tall in the castle wreckage, planning and scheming the remainder of her vengeance when suddenly she heard a snoring coming from behind her. Nightmare turned around multiple times, nearly spinning in place as she looked for its source, and then, after thinking that her mind had been tricking her, she craned her neck around, spying the foal fast asleep on her back.
Furious at first for how the little colt dared to trespass upon her person, Nightmare tried to shake him off, attempting to wrench the foal free from her back with all her magic. To her shock, the foal remained, latched onto her pelt as if glued in place.
“What sorcery is this?” She'd hissed in frustration. “A tiny foal, able to resist mine own arcane fury?!”
The colt merely snored.
The midnight alicorn's gaze seethed with anger, Nightmare Moon trying her best to dislodge the foal that had somehow stuck himself to her back. She rolled about on the ground, scraped herself up against a stone pillar, even flew up to a great height, intentionally diving down and landing flat on her back at full speed.
After her last attempt at that, the alicorn found herself with quite a fierce headache; even godlike creatures from Equestria could only take a sky dive into pure stone so many times.
At first she smiled a wicked grin, the snoring having stopped.
“Ha ha!” she cackled. “At last, We are free of that interloping foal, Our strength has saved Us from that--”
The alicorn found her speech interrupted as a gentle yawn sounded from behind her. Eyes wide with disbelief, the midnight controller of the moon looked behind her, finding the foal still firmly affixed to her back with not even a single scratch on his velvet-like coat.
“How can this be?” She softly, quickly muttered, eyes still wide in shock.
The foal stirred, his big, youthful eyes opening slowly.
He smiled at her as he looked upon her armor-clad form.
“Soft...” he murmured, once more lying down on her back.
“What?” she replied, an eyebrow raised.
“You,” the colt said, nuzzling the alicorn. “Soft fur...”
“Hmph... You know not to whom you speak.” Nightmare snorted.
“Fuzzy...” the colt went on.
“Bah,” Nightmare replied sharply looking away from the foal, “you know nothing of me. The royal pelt is far superior to mere common... fuzz...”
“Still nice...” the colt muttered, promptly falling asleep once more.
“Nice, hm?” Nightmare whispered. “It has been far too long since anypony has deemed me to be so.”
The colt resumed his snoring, rather loudly for someone so young.
“And yet,” Nightmare grimmaced, “BE GONE!”
The roar of her regal, royal Canterlot voice echoed outward through the ruined castle, loose stones shaking in place as they rattled about.
And yet still the foal remained.
Nightmare slumped slightly, the alicorn sitting down firmly on the stone floor. She was quite vexed, and somewhat impressed at the feat this nigh-indestructible foal had managed.
And so we find her where the tale began, stuck in her ruined home, foal seemingly glued to her back.
And she was still far from pleased.
“Foal,” she eventually said, a sarcastic twinge to her voice, “how long do you plan on remaining atop my back?”
The foal stirred, the colt's eyes eventually meeting her's.
“I dunno...” he sleepily replied, rubbing his eyes. “You seemed nice when you appeared before. Your fur is nice and soft too.”
“As you've said,” Nightmare interrupted.
The foal recoiled slightly, calming down after a few seconds.
“A-and that purple pony said something about you m-making the night time and stars and moon and such... and that you were really stuck on the moon for a really, really long time...”
“Yes, what of it?” the alicorn glared.
“W-well, I like the night time,” the colt continued. “The stars and moon look so pretty, and they seem even better now that you're here.”
“Well,” Nightmare said, her tone more relaxed, “at long last, one who can comprehend true majesty. Do make yourself at home in the darkness, foal; it will never leave, not EVER. If others cannot appreciate my work, I will forever hold the night in its place until they learn to keep themselves in their own!”
Nightmare quickly turned her head away, the two not speaking for several moments until the foal, curious as foals are, eventually spoke.
“How's anypony gonna eat?” he asked.
“What?” the midnight alicorn responded.
“Night is nice, but plants need sun to grow. No plants, no food,” he went on.
“Then they all shall starve,” Nightmare growled in response.
“Then there wouldn't be anypony left,” the foal said. “It'd be like being locked up on the moon all over again, oh, unless there was somepony up there too.”
Nightmare trembled a bit, a tear going down her face as she remembered the pain of a thousand years of isolation. Her anger building, the midnight alicorn was forced to a conclusion she really didn't want to even consider; would her night time bring about the end of all life on this world? Would this planet become as empty and lonely as her moon?
Would it become nothing more than another empty prison, only this time one filled with skeletons and graves, mementos of a lost opportunity?
Nearly an hour passed, the foal growing somewhat upset as he watched Nightmare cry softly to herself. Eventually he released his grip on her back, the foal sliding down her wing and crawling up to her side.
“Maybe if you put the sun back,” he said, “ponies would like you.”
Nightmare looked at him, wiping the tears away.
“No,” she replied, a slight tremble to her voice, “they would despise me. They would rally against me, my sister, Celestia, there to lead them.”
“I don't think so,” the foal said. “I think they'd probably understand if you just told them how upset you are.”
“Nopony would ever do that,” Nightmare hissed. “They would would seek revenge upon Us for daring to lock away their beloved Princess of the sun.”
“You think they would?”
Nightmare nodded.
“Hmm...” the foal thought, his mind still young, but rather clever. “Then maybe ask your sister not to do that... revenge... stuff...”
Although his vocabulary was still rather limited.
“You jest,” Nightmare replied. “We have imprisoned Celestia in another realm. Freeing her would surely just invite her wrath.”
“But she's your sister,” the foal said. “She's gotta still care about you, at least a little.”
“And if not?”
“Then... Um...” the foal stammered. “Um... Oh! I'll ask her really, really nicely not to hurt anypony! That should--”
“How naive,” Nightmare groaned.
The foal slumped down, not understanding the word “naive” but still loosely getting its meaning.
“Ugh...” Nightmare sighed, rolling her eyes, “No harm meant, foal. And... your love of the night is... appreciated. It is unfortunate though that you do not yet understand such matters. It is far too complex for the young mind.”
“Grown-up ponies can be so stupid,” the foal muttered, somewhat aggravated.
“Excuse me?” Nightmare asked.
“Hrrmmm... Grown-up ponies make all these dumb rules and stuff and don't tell anypony else what they mean!” the foal sounded rather cross. “Why tell us colts and fillies to do good stuff if you grown-ups just go ahead and do bad stuff anyway?! It's stupid!”
Nightmare was taken aback slightly by the foal's forwardness. Never in thousands of years had the night time alicorn ever come across a foal so brazen. Nightmare felt as though she should be mad; after all, how dare a young foal speak out in such a way against a Princess of Equestria.
And yet... he might have a point.
“And my name isn't 'foal',” he went on, less loud now but still quite annoyed, “it's Sunny Day.”
Nightmare couldn't help but laugh slightly at the irony.
“Oh, the irony,” the alicorn chuckled, “a foal named for the day seeking refuge with the night.”
The foal, Sunny Day, grumbled slightly.
“Take no offense, please,” Nightmare said, “We mean you no insult; it merely amuses Us.”
Sunny Day just frowned.
“If you're sorry,” he said, looking her straight in the eye, “then bring your sister back.”
“Setting ultimatums now?” Nightmare raised an eye. “My my, you are brave. Not many would be so daring around Us.”
“If she tries to be a meany and hurt you, I'll stop her for you!” the foal said.
Nightmare couldn't help but laugh. This foal was either very brave, very foolhardy, or very, very stupid. No ordinary pony could ever survive the wrath of an alicorn.
Then again, he did manage to somehow survive hers.
“It's been a thousand years anyway,” the colt went on. “All those bad ponies who hurt you a long time ago are gone now. There are nice ones now who like the night time.”
Nightmare thought to herself for a few minutes, pondering the foal's words over and over again. Perhaps the foal, even if only a little bit, was right.
Reluctantly, the midnight alicorn's magic lit up the room, the entire stone area glowing with a purple and blue haze. With a flash of light visible from even beyond the castle walls, a tall, regal alicorn appeared, her coat a pure white with broad wings, a majestic horn atop her head, a sun-shaped mark on her flank, and a mane and tail made of a flowing rainbow.
She looked at the two, softly at the colt and more sternly at the other alicorn.
“Luna,” she spoke, sounding more sad than angry.
“Celestia,” Nightmare replied with a nod.
Several minutes passed, a loud creaking noise soon filling the room as massive wooden doors swung open. Six mares galloped in, concern on their faces as they made their way up to the two alicorns and their young companion.
“Princess Celestia!” The purple one nearly cried, overjoyed to see her teacher once more.
“Twilight,” the Princess of the sun replied, smiling upon her student. “I see you took my advice. These would be your new friends, I take it?”
The others bowed, relieved to see their Princess unharmed.
“Hey,” Sunny Day piped up, “what about our deal?”
The others looked at the foal, they, too, shocked that someone so young would talk to a Princess in that way.
“It's alright, everypony,” the solar Princess said, “he speaks the truth, we do have a deal, and yes, I agree to it. My sister is not to be punished for her actions as long as you, Twilight, and her friends, help her learn how valuable friendship can be, and that even though there were those long ago who would look down upon her, those days are long gone.”
Celestia sat down next to her sister, close enough so that only she could hear.
“And you'll find, dear sister, that there are plenty who would love to be called your friend.”
Nightmare nodded, a tear of joy this time forming in her eye.
Years went by. Twilight and her friends grew closer, eventually culminating in Twilight's ascension to alicorn-hood herself. Nightmare kept her tall, black-coated form; a memento of the past, but also as a symbol of her own personal growth, although still responding to “Luna” as well as “Nightmare Moon”. The foal, Sunny Day, eventually grew, becoming a strong stallion and part of Nightmare's personal guard, the two still remaining close friends.
One day though, or rather evening, Nightmare couldn't help but ask her night guard captain one question that had lingered on her mind for a very, very long time.
“Captain Day,” she asked.
“Yes, my Princess?” Sunny Day responded.
“Don't worry, Sunny Day; no formalities for now,” Nightmare smiled.
“Okay,” the guard captain said with a small laugh, “what is it, Nightmare?”
“When we first met,” the alicorn asked, “how did you hold on to me so tightly? And, even, well... survive?”
“Well,” Sunny Day grinned, “my mother and father always did tell me I was stronger than most colts.”
Nightmare looked slightly confused.
“In short, I have no idea,” he laughed.
The two laughed a bit after that, each soon returning to their official duties. Friends can come from everywhere, and age is not necessarily a boundary for it. And take care as well; you'd be surprised just how knowledgeable someone younger than you can be.
Also, foals can be annoying, but also quite friendly.