One day, Discord was bored.
He was bored with Fluttershy's tea parties, bored with the mundane everyday life, and bored with the agrarian ideal.
He was truly, utterly bored.
I suppose you could call him dis-bored.
So he floated around Ponyville looking for something to do.
Something that would liven up his day.
He thought about going to Twilight and annoying her, but she was busy preparing a speech for a magic convention in the Crystal Empire in three days and wouldn't be back for a week.
And Discord didn't like going to the Crystal Empire. The Crystal Ponies there looked at him like he was a lingering remnant of Sombra the ponies hadn't yet gotten around to cleansing from the world. They looked at him as though he were nothing but a bad smell or a cruel memory best left forgotten.
And to make matters worse, Cadence had once told him that if he ever unleashed chaos on the Crystal Empire or did anything to harm her family(and in her eyes, the entire Crystal Empire was her family), she wouldn't need the Elements of Harmony to freeze him in time, leaving him in the past while everyone else continued to move forwards in time.
How cruel! You never wanted to mess with Cadence or her family.
Discord continued to float through Ponyville, looking for something to do.
When he saw Rainbow Dash zipping around town, a rainbow trail and an orange filly on a scooter struggling to keep up in her wake, he decided to watch her for a while.
"Keep up, squirt! You've almost beaten your best time!" Rainbow Dash shouted over the whipping winds of her breeze.
"I'm trying, sis!" Scootaloo shouted.
Sister...
That gave Discord an idea.
He began to plan and plot and scheme all day.
That night, he snapped his fingers before going to bed, his almighty magic of chaos rewriting reality how he saw fit.
He added a new actor to this grand stage of life.
And the next day, his plan bore fruit.
It was a bright sunny day in Equestria, and Rainbow Dash found herself awakening to find a weight on her numb wings. Looking back, she saw that she had been trapped in the iron grip of her twin sister's furry bone-white hooves, and her own blue wings had been trapped and squished to form what her white-furred sister used as a pillow.
A white Pegasus, that was what she had for a sister. A white Pegasus whose big clumsy wings started out white, only for each feather to change colour along its length. The tip of each wing feather was a different colour, which meant each and every colour of the rainbow was represented in her wings.
Where Rainbow Dash's eyes were cerise, the white Pegasus's eyes were a full circular spectrum of the rainbow. And their Rainbow-coloured manes were exactly the same in all respects, only differing in one regard. Rainbow Flash's mane was a bit longer and tied up in a nice ponytail. But aside from that one respected regard, all respects regarded their manes as the same.
Struggling a little, Rainbow found that she couldn't escape. Not without waking her sister up.
So she decided to gently shake her sister.
"Not so rough, love," She giggled softly. "Oh, Mr Bear, what will my father think?"
Rainbow Dash rolled her eyes. Was that hopeless romantic dreaming about being in love again? Her wings started to tingle, she knew they'd need to be stretched out soon.
"Get off me, sis..." Rainbow softly whispered to her sister, shaking her harder until the sleeping dear's expression turned less serene.
"Five more minutes," Her sister asked, hugging tighter and nuzzling into her wings.
But Rainbow was done asking.
"Sis, let go!" Rainbow stretched out her wings, breaking her sister's grip and pushing her off her bed.
"Oof!" The white Pegasus fell onto the soft cloud floors, bouncing back up like she was on a bouncy castle, landing on her hooves. Being pushed didn't seem to bother her. "This is so much softer! Why don't we sleep on clouds?"
Rainbow walked towards her closet and threw it open, sorting through hundreds of dresses she didn't remember seeing before. "Because they're way too soft! It just doesn't feel right to me." Said her voice.
Rainbow Dash remembered that she didn't wear clothes usually and none of the dresses here were hers. Only the old Wonderbolt Jumpsuit on the closet floor belonged to her, and she didn't feel like wearing that today. So she closed the closet and looked back to her sister.
"Is that why I'll sometimes catch you lazing around on top of houses, or on Applejack's trees?" The white Pegasus smiled.
"Hey, I don't laze! I just rest my eyes sometimes, that's all. Anyway, you've got to stop sleeping with me! We aren't fillies any more. Don't you have your own bed?" Rainbow asked.
The white Pegasus raised an eyebrow.
"Oh, right." Rainbow Dash smiled sheepishly. "Want to go and get a bed today?"
"Sure! Then you can introduce me to all of your friends."
"You've never met my friends before?" Rainbow Dash asked. Something about this felt wrong, and she pondered reality for a second. Then she stretched her wings out and smiled. "Well, we've gotta fix that! Come on, Rainbow Flash!"
The white Pegasus stretched her own rainbow-tipped wings, her big rainbow-coloured eyes sparkling brightly. "I'm ready, Rainbow Dash!"
The two flew out of their house's bedroom and soared into Ponyville like eagles, not bothering to lock their houses because you never needed to lock a house in Equestria. Even the meanest ponies of all time were still good at heart, and would never sink so low as to rob another pony! You had to be truly rotten to the core or just plain raised wrong before the magic in the air would let any creature think of doing anything truly evil.
The duo enjoyed the breeze, but Rainbow Dash was shocked when her twin sister sped up and went absolutely wild in the air, chaining together flips and spins and nose dives and twirls like a filly discovering the joy of flight for a first time. She flapped harder, going faster and faster, flying head-first through clouds so the bursting moisture would soak her body and feel so cold and amazing and most importantly, hide the tears forming in her eyes as the joy of being alive hit this pony for the first time in her life.
Rainbow Flash zipped past trees and over flower gardens, flying into Ponyville lake and flying back out with a fish in her mouth, flying over Fluttershy's cottage and spitting the fish onto the ground for one of her pet wolves to eat.
When Fluttershy looked up to see who had dropped the fish, all she saw was Rainbow Dash trailing after her flying friend.
Rainbow Flash flew all over the world without a destination in mind, all she could think of was her love for nyooming around the sky.
Rainbow Dash flapped her wings hard and zoomed ahead of her sister as they both flew close to Canterlot.
"Hey, daredevil! Ponyville is this way!" Rainbow Dash told her sister, and led the way back to Ponyville.
Rainbow Flash couldn't wait to meet all of her sister's friends.
And Discord, watching it all from afar with a pair of binoculars, couldn't wait to see how long it would take for the ponies of Ponyville to notice the little prank he'd played upon them.