Author's Note
The Library has been called by several different names in different Hasbro and Hasbro-authorized properties and products. See https://mlp.fandom.com/wiki/Golden_Oak_Library .
(You can see the Library's three-in-one front door if you look closely at the screencap in this story's cover image.)
Fanon Enforcement
Ponyville's resident human knocked on a door set into a tree. "Yoo-hoo!" he said. "Anypony here?"
A minute later, the bottom left and bottom right portions of the door opened. "Sorry," a little dragon said, looking up at the human's face. "I'm short. I can't reach the latch or doorknob for the upper door."
"And that's another thing," the human said.
"What? You don't like that I'm short? Maybe I don't like it either."
"It just doesn't make any sense that Ponyville's public library would have THREE doors in ONE doorway. What's the point?"
"I dunno." Spike shrugged. "I didn't do it. The library was this way when Twilight and I moved in."
"But why would anypony have built the Golden Oak Library this way?"
"Golden Oaks," Spike said.
"What?"
"Golden Oaks. Plural."
"But it's just one tree. How can...oh, forget it." The man got down on hands and knees to crawl through the space under the door's top half. He stood to look at the bookshelves. "I used to fantasize about someday being sucked into Equestria and meeting everypony and getting to live in a wonderful world of magic. But so many things are just WRONG! Even the Golden Oaks Library."
"Books and Branches Library," Spike said.
"But just a minute ago it was...never mind." The man leafed idly through a book titled, "Forbidden Magics (DO NOT READ!)" He said, "So many things about this place just don't fit my headcanon."
"What? Your head conan?"
"No, you silly lizard. A CONAN would be a barbarian." The man's eyes skimmed over a page. "Of course! That's the answer!"
Spike quirked his eyebrows.
"The spell on this page!" the human said. "I can cast a spell across the dimensions, to summon a fierce barbarian from another world. Some muscular man with a big sword and fur trousers, or armor-plate swim trunks."
"Another world?" Spike asked. "Do you mean, like, another planet?"
"Sure! I don't care. Earth, Mars, wherever. And my spell will fill his mind with an exact duplicate of my own precious fanon knowledge about MLP. My barbarian will understand the truth! That everypony and everything should be exactly the way I think they should be."
Spike frowned. "Are you sure that's a good idea? When Twilight tried a mind control spell, it didn't go well at all."
"Unlike her, I understand how things should be done in this world." The man reached into a human-style saddlebag on his hip (definitely not a purse or a fanny pack.) He pulled out a plastic horn with a long elastic strap (definitely not a party hat) and put it on his head.
Light swirled around the hat-horn, and struck the library floor, where it...disappeared into a momentary dark hole, into another dimension.
"Wow!" Spike said. "That sure was...something. Now what do we do? Run away?"
"No," the man said. "Now we wait."
***
Later that morning, the man had returned to Fluttershy's new Animal Menagerie and Sanctuary, where he lazed about while receiving free room and board for being some kind of weird new animal never before known to pony science.
He was sitting in a lawn chair on the grass outside her cottage, reading a comic book, when Fluttershy came running. She said, "It's terrible! There's another one of you, I think, maybe." She blinked. "Or maybe it isn't one of you? I really can't tell."
The man sighed. "Is it in town?"
Fluttershy nodded.
"I'll go take a look." The man laid his comic book down, got out of his (actually Fluttershy's) chair, and walked towards the center of town.
***
"This is wonderful! he said. "A genuine human barbarian! Look at that precise swordwork! And those high quality...steel plate short pants."
"Is that what he's wearing?" Fluttershy said from somewhere farther back.
"Of course! Watch him slicing the heads off those non-canon compliant flowers. Don't you admire his strength and skill?"
Fluttershy winced as the barbarian invader used its sword to hack and slash a sales display of daisies. The flower sellers to whom the flowers belonged had, of course, protected themselves from errant swordplay by collapsing to the ground, and taking turns shouting "The horror! The horror!" just so everycreature would know they were down there and maybe please not step on them.
"I'm sure his helmet is very nice too," Fluttershy said. "It looks almost Fleecian or Roaman. But why is he so angry?"
The scantily clad, sword-wielding creature slashed at an awning overhead, exposing anypony who should walk underneath it later to harsh, unforgiving sun.
"Why is he so mean?" Fluttershy complained.
"Well...he must be my head-conan. A barbarian from another world, who destroys anything inauthentic. The awning's color must not quite match one of the My Little Pony episodes, or maybe one of the movies."
Sweetie Belle, Apple Bloom, and Scootaloo ran directly towards the havoc. "Cutie Mark Crusaders Evil Creature Tamers Go!" they shouted as one. Sweetie Belle's horn spewed a small tendril of magic at the invader. The tendril wrapped around the sword, tugging at it.
"Inappropriate!" the invader shouted. "Sweetie Belle is girlish and timid. She would NEVER confront a muscular barbarian larger than herself."
Fluttershy snorted. "Has he ever HEARD of the Cutie Mark Crusaders? Timid is NOT the word for them."
The creature swung a sword at Sweetie Belle, striking at her neck. Instead of blood, a "CLANG!" was his reward.
"By the Lost Sea of Mars!" the creature shouted. "You are not a proper unicorn filly at all, but some kind of infernal machine! NOT canon!" He swung again and again, making the town square sound like a blacksmith's convention, or perhaps a drunken hobo trapped inside an extra-large commercial trash container. He shouted remarks like "Begone, metallic witch!"
Beside Fluttershy's pet human, Twilight strolled up. "Hello, Humie. Do you know anything about this?"
"Um..." the human said.
"I thought so," Twilight replied.
"This is wrong!" Humie complained. "You're a nerd without even the smallest trace of social competence. You shouldn't be ABLE to notice that I'm hiding something."
Twilight poked his side. "Ponies change, my little Humie. And everything that happens in what you call 'canon' implies that more things happen the rest of the time, in what I think you've called 'off camera.' Otherwise, between the times that you call 'episodes,' everypony would starve to death, or freeze, or just die of boredom or something else like that."
"But...how do you excuse Sweetie Belle being swordproof? That is NOT APPROPRIATE AT ALL!"
Twilight snorted. "And a human walking around Ponyville IS appropriate? Look at yourself, Humie. What possible excuse do you have for being here?"
"Megan, G1. Megan provides a precedent, which justifies my being here too."
"Your name is NOT Megan. Your being here can mean only one thing, Humie. Don't you know that?"
The man glared at his little purple interrogator.
Twilight answered her own question. "This world is AU. Alternate Universe. Not what you call the original at all."
"NO!" the man shouted at the heavens. "NOT an AU! No! No no no no no no no no AAAAAAUUUUGH! A fate worse than DEATH!"
Twilight chuckled. With one hoof she pointed at the swordsman, whose helmet had now slipped forward under a pegasus' barrage of stones from above, blocking his vision as he swung and chopped at nothing. Twilight shouted at the sword-wielding invader, "Hey, you with the sword! Over here! There's a HUMAN in Equestria! That can't be right, can it? Straighten your helmet so you can see, and take a look!"
The new arrival reached up with one hand to straighten his helmet. "You!" he shouted at Humie. "You do NOT belong here."
"Wait!" Humie pleaded. "You must not harm me! I cast the spell that brought you here and gave you your special abilities!"
The barely-armored swordsman squinted at Humie. "I should have thought as much. You have the weak, pasty look about you of one of those cowardly scientist-sorcerers who, tales say, live in hidden caverns beneath Barsoom's surface."
The fierce fighter spat upon the grass. "If your spell brought me here, then perhaps by defeating you, I can win my freedom and return home!" He charged at Humie, who stood frozen in place. Two muscular arms lifted the sword high, and brought the blunt hilt down against the top of Humie's head, as if to knock the Earth human unconscious.
In a flash of light, both the swordsman and Humie disappeared.
Everypony stared at the suddenly empty patch of grass where the two humans had been.
Applejack, approaching from the other end of the marketplace, was the first to speak. "What happened here?" she asked.
"I'm not sure," Spike said. "Maybe comedy?"
One of the flower sellers sat up. "A strange creature from another world appeared, and attacked ponies with a sword, threatening to destroy anypony who didn't meet its ideas of how Equestria should be. Obviously horror."
"You always say horror!" a passerby complained.
"That doesn't mean horror can't always be the right answer!" retorted the flower vendor's sister. "Equestria is FULL of monster attacks and threats to enslave all the ponies or turn us into strawberry milchcows or something like that. Equestria IS full-time horror, twenty-four seven. Whenever the place seems peaceful for a moment, that isn't SAFETY! It's just...building suspense."
"What even WAS that thing?" Rainbow Dash asked from above, where she had been dropping rocks on the Martian helmet earlier to see how loud a 'bong' noise she could make make (and to distract the invader from attacking Sweetie Belle.) "Seems pretty random to me."
Fluttershy snorted. "Humie was always complaining about how his bed wasn't comfy enough, or me having a love affair with another pony wasn't canon, or Angel Bunny didn't taste good the day I was away and Humie caught and roasted the little rabbit." She snorted again. "A day with Humie around was always full of drama."
"Doesn't a weird creature from another world make it sci-fi?" Scootaloo asked. "I'd ask my parents, but...you know."
Rarity shrugged. "Nopony seems to know for sure. I suppose the true answer is a mystery. But I do know one thing for sure."
"What?" Sweetie Belle asked in a robotic voice.
"Humie is finally gone, and Ponyville should be a lot more peaceful from now on."
