Princess Celestia rose before the sun. Quite literally, as she was the one who rose the sun. Her golden magical glow lifted the sun above the horizon before she even noticed anything was amiss.
The first thing was that Celestia was about a third of the height she had been last night.
The second thing was that she didn't feel like thinking about it anyway.
With what curiosity she could feel, Celestia hopped on her desk to look in the mirror. What looked back at her was the most beautiful, majestic creature ever to appear on the world.
Celestia was a chicken.
Everypony else was also a chicken. It was chaos.
In Ponyville, chickens of every color, some with little horns, flapped about, bawking loudly. They pecked the ground and each other. One displayed her advanced, almost coleslaw-level intelligence by getting stuck in a fence. It was glorious. It was majestic. It smelled really weird for some reason.
Twilight sat outside her castle door. How she had gotten there was a mystery, considering she had the intelligence of a cardboard box and couldn't have possibly navigated her way outside. She also seemed way too inactive to have wandered out on her own, but there she was. Sitting directly outside of the door, she was doing something resembling taking notes by scratching the dirt.
In Canterlot, many chickens in fancy clothes walked around, clucking a lot and pecking the ground randomly. Several fell off the edge of the city and flew through the air like rainbow kites. They sat on tables and went through doors. It was lovely.
The next day, everything was normal, so Twilight wrote a friendship letter.
Dear Princess Celestia,
Yesterday everyone in Equestria was a chicken. I am still investigating the cause. But I don't feel like writing right now, so bye, I guess.
Signed, Princess Twilight Sparkle
She didn't notice the purple chicken feathers that littered the floor.