Backstory: This story was written because I told my Twitter feed to stop it with the pony drama or I'd write G3.5 clopfic. They didn't. So I wrote G3.5 clopfic. This story, unlike many things the Internet blames her for, is in fact entirely Purple Tinker's fault. More backstory/meta in the descriptions.
One nice cool spring afternoon, Pinkie Pie trotted along pretty much the only road in Ponyville. She was in high spirits since she is heading to a tea party with all her friends. "I hope I'm not late," thought Pinkie Pie, "it took forever to pick out which skirt to wear". It indeed took a long time for Pinkie Pie to choose the blue skirt that she was wearing, despite only owning about three skirts. If only she had the fashion sense of Rainbow Dash! What's more, since ponies somehow have freakishly small waists compared to their legs it took almost an hour to put the skirt on. Well, at least she was more flexible than she has ever been at this point.
After an amount of time equal to exactly one internal monologue about tea parties, Pinkie Pie arrived at a hat-shaped building in the middle of Ponyville. This building has never been here before. But it's okay, that happens all the time. Buildings just pop into existence in Ponyville whenever some god-like entity called Marketing demanded it, and then Pinkie Pie and all of her bestest friends would go to said building and dress up and have tea parties. Pinkie Pie trotted happily into the hat---a sign on the front called it the Hat Box Café---and prepared for an afternoon of delightful and pointless existence.
"Surprise!" yelled Sweetie-Belle, Scootaloo, StarSong and Rainbow Dash, four of six of Pinkie Pie's bestest friends. Of course, it was hard to make friends with other ponies since Ponyville only has seven ponies. Regardless, Pinkie Pie is everypony's best friend because that's what she does. She is everypony's bestest best friend forever. She also likes cupcakes and is secretly wondering when she can stop being existential and just eat some bucking cupcakes already. Anyway, Pinkie Pie pretended to be surprised because, honestly, they do this surprise party thing all the time.
"You got me, girls! Now, let's eat some cupcakes!"
The ponies all laughed politely, but not too much, and then they proceeded to a plastic table where cheap plastic cups and plates are on the table. However, they are empty.
"Oh right, I was supposed to bring the cupcakes and tea!"
"Oh Pinkie Pie," Rainbow Dash said, "it's okay. We can eat my cupcakes instead."
With a magical twirl, Rainbow Dash rips off her plastic skirt and flings it through the air. It made a rainbow. Because her name is Rainbow Dash and her butt has a rainbow on it. Rainbow.
"Mmm, Rainbow Dash always undresses in style," said Scootaloo, who totally does not have a crush on Rainbow Dash what-so-ever because Scootaloo will only ever love another pony who is also adventurous and like to go outside and play with proper adult supervision.
"Hey, I can do that too!" StarSong was unhappy that Rainbow Dash got all the attention. The purple pony needed attention too. In fact she needed it all the time. One day, she mused, the world will revolve around a beautiful purple pony. She tried to rip her clothing off in a spectacular fashion but since her cutie mark was not about fashion she failed miserably. She ended up tangling herself up for an obligatory bondage reference.
"Ha ha, StarSong, you're such a silly pony," Pinkie Pie said, "let me help you up!" Pinkie Pie is everypony's best friend, so she must be loyal even though she also laughs at her friends. However, Pinkie Pie did not notice Sweetie-Belle, the smallest pony of them all, and tripped over her giant head.
"Sorry, Pinkie Pie, I should have worn a giant hat today to warn others of my small stature!"
"Wow," Scootaloo mused, "stature is such a fancy word! Has Cheerilee been teaching you big words again?"
Pinkie Pie tumbled into StarSong and the two of them landed awkwardly on the floor, with Pinkie Pie staring into the abyss that is StarSong's blue eyes. It wasn't hard considering that her eyes were kind of huge. In fact, this happened a lot. Rainbow Dash signed and tried to pull them apart. But since she was not particularly athletic herself she ended up falling into their embrace. And then, somehow, as if controlled by a giant controller held in Scootaloo's hooves, the ball of three ponies rolled into Sweetie-Belle and Scootaloo. They all stared into each others' eyes for what seemed like twenty-eight-point-two-six-one eternities.
Suddenly, Pinkie Pie remembered. She had to feed her friends "tea" and "cupcakes" because she was everypony's best friend. So they had sex. Lots of it. Because there's really nothing else to do in Ponyville. Their bodies writhed together in some incomprehensible form that cannot be spoken of. If they were to be described to you, dear readers, your brains would implode because you have a basic understanding of anatomy.
After the massive pony orgy the pony pals all laid down together and looked at the moon. The beautiful moonlight became their blanket as they failed miserably at snuggling due to their awkward pony bodies. As the mooned looked down upon them Pinkie Pie thought that they sure learned something today, though she was not quite sure what that lesson was because they forgot to write it down.
Nine months later, Pinkie Pie was sitting in yet another cupcake shop. Lacking hospitals or even stalls with hay inside this was the only place that the ponies could bring a sick friend to. Cheerilee tried very hard to cheer up Pinkie Pie with stories about cupcakes and friendship, while Toola-Roola made the room extra pink. However, nothing could stop Pinkie Pie from being in pain.
Suddenly, Pinkie Pie let out a scream. All six of her bestest pony pal friend buddies surrounded her out of concern. Before them and between Pinkie Pie's hind legs laid seven hideous monstrosities. They were squishy, proto-pony forms sitting upright. Their manes were not fully formed, and they were each wearing a dress. Scootaloo touched one of them---the one that looked strangely like her.
"She's so… soft…"
Those were Scootaloo's last words before the newly born "Scootaloo" brutally murdered the former Scootaloo. The other baby ponies joined in, snapping the necks of the older ponies easily because their necks were impossibly thin compared to their heads.
"Friends?" a soft pink newborn pony asked.
The seven baby ponies got up and pranced around, looking for dresses and pianos and picnic baskets because they were programmed in their very DNA to do so. One day, they will grow up and replace to old Core 7 ponies. Such is the way of life in Pony---
"We have had enough of this!"
Suddenly, lightning flashed and the moon itself appeared before the seven hideously terrifying monster-ponies.
"This life you lead, it is… disgusting," said the moon. Her voice was so loud that it pushed Sweetie-Belle against the wall and killed her because Sweetie-Belle was small. "We shall cleanse Ponyville of your vile existence!"
With that, Princess Luna smothered the other babies with her Royal Canterlot Pillow, and then with the help of her sister Princess Celestia they reshaped Ponyville into something that wasn't horrifying and insane.
And that's how Equestria was made.