"Wait," said Sunset. "You're saying Old Holly Hooves isn't real?!"

Her friends all looked at her strangely.

"...Well. Yes." Twilight nodded. "I mean, it's scientifically... I guess not impossible, strictly speaking, but—"

"I put up a stocking every year! I've been doing it since I was a filly! And every year, it's been filled!"

"Generally it's the parents that—"

"Every year, Twilight! Including the ones I was here!" Sunset shrugged. "Granted, I tended to get coal, but I assumed that was just a quirk of this reality."

"...oooooookay. That's... actually kind of weird."

"Hey," Pinkie spoke up. "If you're kind of omnipresent, can't you just, I dunno, go find Old Holly Hooves yourself and ask him if he's real?"

Sunset blinked. "Well... maybe? He's probably busy around this time of year, though. I'll just wait till summer."

Author's Notes:

From the thread:

FanOfMostEverything: I love the idea of Sunset putting up a stocking even during her less pleasant phase, but would the human world still have a reindeer running things?
Masterweaver: This is one of those things I don't want to answer because it's funnier that way.