"Of COURSE I can tell you which came first," Twilight Sparkle insisted, arguing in the middle of a Ponyville street. "Ponies and all other four-limbed creatures that live outside the sea evolved from fish. And fish hatch from eggs. So the egg came BEFORE the chicken."
A little orange pegasus filly had been trotting past, but now she stopped. "Are you saying rude things about me being a chicken?"
"Of course not," Applejack said. "It was a misunderstanding."
"I was just explaining where ponies came from," Twilight said. "We used to be fish."
"Not true!" Scootaloo complained. "Chicken, fish...I've never been any of those things, and grown mares ought to know better than to say so!"
Twilight shook her head. "I don't mean you personally, Scootaloo. I mean our distant ancestors."
Applejack scoffed, "Ah reckon you're full of...applesauce, Twilight, and not the good kind neither. Ah know ponies have ALWAYS been ponies--"
"Um," Twilight tried to interrupt.
"Ever since we were created directly from horses!"
Twilight blinked. "You think we used to be HORSES?"
"Well of course! Where else would we have come from?"
Scootaloo piped up, "I heard we evolved from worms." She scratched her head. "Or maybe parasprites? I'm not sure."
Twilight shook her head. "I cannot BELIEVE the sad state of Equestrian education. The simplest facts of life, and hardly anypony but me seems to be properly informed about them."
A unicorn with a pinkish-lavender coat strolled up. "What doesn't anypony seem to know, Twilight? Because I can tell you, my education is going GREAT. I just learned a new time spell!"
"Oh no!" Twilight said. "Not again. Starlight, I remember how you almost destroyed Equestria a dozen times or more, the LAST time you toyed with time travel."
Starlight smiled at Twilight, trying to look innocent. "This is completely different! This spell isn't a time TRAVEL spell, it's just kind of a...time NOT traveling spell. And it's temporary. Wait a day or two after you cast it, and everything goes back to normal."
Twilight shook her head. "No. Bad Glimmer." She levitated a spray bottle and squirted a stream of water at Starlight Glimmer, who opened her mouth wide.
"Thanks! How did you know I was thirsty?"
"BAD GLIMMER!"
"Waitasec," Applejack said. "Ah want to hear about how this latest spell is supposed to be different."
Starlight gestured with one forehoof, pointing at a flower growing in front of Applejack. "Suppose you want to see what that flower used to look like, before it opened. Just cast one little spell, like this..." Her horn glowed, zapped the flower, and it changed from a large, open bloom into a little bud. "See? It's easy. And by sometime tomorrow, the bud should change back into its regular flower self. So there are no drawbacks! No problems at all! Perfectly safe! And if you step up the power a little, it's also a great way to settle paternity cases by turning a pony into her own parents for a day!"
"Hmm," Twilight said.
"Do it to Twilight," Applejack said.
"What?" Starlight asked.
"Twilight says ponies all used to be fish, long ago."
"Our ANCIENT ANCESTORS were fish," Twilight corrected. "But you and I, Applejack, have NEVER been fish. Unless you're your own great-great-so-many-greats-grandmother."
"Jokes about me bein' my own great grandma are NOT funny!" AJ stomped her hoof. "You know redneck jokes like that are just prejudice. You shouldn't say things like that!"
"I didn't mean it as a redneck joke!" Twilight protested. "You should know me better than that."
"An' tryin' to tell me ponies used to be fish. You're just makin' fun of me, is all you've been doin'. Playin' me for a fool." Applejack poked Starlight with one forehoof. "Ah DEMAND ya settle this once an' for all. Use your spell on super high power, an' turn Twilight into her long-ago ancestor!"
Starlight chuckled nervously. "Maybe we can compromise? What if we try it on a...a dog or something? Somecreature who isn't a pony?"
Twilight glared back. "Don't act like dogs don't matter! Sometimes SPIKE is a dog!"
Starlight blinked. "I didn't know. May I try my spell on...a tiny lizard? An earthworm? What kind of creature WOULD be ok?"
"Hmmph. Try it on Applejack. She's so sure her family's always been ponies, she shouldn't even mind it at all."
Starlight shrugged. "Ok, if you say so." A magical glow swirled brightly around her horn. "You might want to cover your eyes against the flash. Turning somepony into her own distant ancestor is going to take a LOT of magical power--"
"No!" Twilight shouted as she reared up on her hind legs. "I was kidding. It was just a joke!"
"So you WERE making fun of me!" Applejack shouted back. "Why, Ah oughta..."
Starlight grimaced. "Oh my. This spell feels very...potent. I don't think it's an easy one to just stop partway through." Just as Twilight tried to push Starlight aside to redirect the spell's aim away from Applejack, a blazing bolt of magic shot out of Starlight's horn.
Everypony blinked, half-blinded by the spell's flash.
"Twilight?" Applejack said. "Ah'm still a pony. So that PROVES--"
Beyond Applejack, a slapping noise like a wet, wriggling fish interrupted. "Darlings? My hind end seems to have been struck by somepony's poorly considered spell."
"Yes!" Scootaloo said. "I'm NOT a chicken! Finally somepony ELSE is gonna get called some creature other than a pony. NOT me."
Applejack frowned at Rarity. "Rarity? Your backside is lookin' kinda...fishy."
"Yes, dear. How attentive of you to notice. I appear to have become a merpony."
Scootaloo agreed, "You sure are."
Applejack told the half-fish fashionista, "Starlight says in a day or two you'll turn back to your regular self. But for now, we should get you into some water right away, before you dry out." She ran over to Rarity, carefully gripped her pale friend in her mouth, and flipped the merpony up onto her strong earth pony back. "Rarity, let's you an' Ah go swimmin'. Maybe in Ponyville Pond?"
"Thank you! I accept your kind invitation."
Applejack carried Rarity away, stepping carefully.
"So as you can see," Starlight Glimmer said, "A good magical spell can--"
"Make us look like reckless foals?" Twilight asked.
"I was GOING to say, can settle arguments about the origin of ponykind. Just one little spell gives us so much scientific knowledge. Isn't it marvelous?"
Twilight sighed. "Just don't do it again."
"Oh, of course not!" Starlight winked. "Unless you TELL me to do it, again, of course."
Scootaloo picked up the spray bottle in her mouth. She squirted both Twilight and Starlight. "Bad unicorns! Naughty unicorns! It's not fair. Why do I have to be the responsible grownup pony around here?"
