Xlestia rubbed her forehoves with glee. Soon, she would establish the Barrier she had planned out, which would counter the human technology easily. Conquering this desolate wasteland would be -
"Are you Princess Celestia?" An unfamiliar voice interrupted.
Xlestia blinked. Turning, she saw somepony she had never seen before. The Earth Pony stallion had a dark grey coat with a dark brown mane, and... no cutie mark? At that age?
"Are you Princess Celestia?" The stallion asked again.
"Yes...?" Xlestia answered in confusion.
The stallion nodded, reached its forehooves around her head, and cleanly snapped her neck. Dead, Xlestia collapsed to the floor.
The T-850 (Pony variant) blinked. Mission accomplished. Future averted. Returning to Skynet.
Warhammer 40,000 Suggested by RoadRunneR, master109, and dragonjek
It was funny, Xlestia mused. From her scrying, she would have thought that conquering humanity and converting them to ponies would be harder than this. Sure, there were a couple billion humans who all resisted until their last breath, but they were handled with superior magic.
It was weird - she had won, so why was she waiting for the other horseshoe to drop?
Little did Xlestia know that the small Hive world she had converted was literally too small for mainstream humanity to notice, let alone avenge. The human race was simply so large that the addition of 12 billion ponies still didn't make her anything other than a negligible threat.
The true threats out-powered the sun princess by so much it was worse than comparing an ant to a space hulk.
And so, the successful conversion of a planet went completely unnoticed by the Imperium of Man.
Neo-Equestria was later devoured by a Tyranid Splinter Fleet. Other than the Tyranids, no one noticed.
Sailor Moon Suggested by DreadedCandiru2 and Shadow Man
It wasn't often that someone else was able to make Xlestia feel awkward with how girly they were, but this "Sailor Moon" girl had managed to out-girly the flying unicorn princess.
Somehow.
"Ponies are for young girls to fawn over, not a fate for the human race! In the name of the moon, I will punish you!" The mini-skirt and leotard wearing girl yelled, striking what was quite frankly a silly-looking pose.
There was a beat as Xlestia tried to recover her verbal momentum.
"Are you sure? I think you would make a very pretty unicorn." Xlestia managed, then cringed. There's no way she'll fall for that!
Sailor Moon blushed, tugging the edges of her skirt down. "Aw, you mean that?"
Xlestia started, then hastily continued. "And we could brush each others mane, and go running in the woods, and watch the sunsets together..." Xlestia felt nauseous inside from the plain bad prose she was sprouting.. but hey, what ever works, right?
Moon continued to fidget. "Well, I guess that's not so bad really... oh! OH! I WOULD GET TO MARRY A PONY!"
"Yes! Yes you would!"
"Okay then, sign me - "
Loud wing-beats sounded as another alicorn alighted in the clearing. "Annnndddd I'm going to stop you there." He stated quite clearly.
Xlestia looked up in surprise. Not only was this alicorn a horse alicorn rather than a pony alicorn (and thus quite a bit larger than her) but she knew this alicorn. "Helios?! What are you doing here?"
"Celestia... do you remember where we live?" The alicorn answered with as much dignity as he could manage, considering that Moon had started stroking his fur.
"Equestria?"
"Which is where?"
"In... the... dreams of children?"
Helios nodded approvingly. "Now, what do you think will happen if you make the dream world and the waking world the same?"
"Not good things?" Xlestia weakly guessed.
"Not good things." Helios confirmed. "It's a nice idea Celly, but it just plain can't work."
"Aww." Moon whined. "Does this mean I can't be a pony?"
Helios nuzzled the girl gently. "You can always be a pony - just drop by Elysion the next time you dream."
"Okay" Moon said, brightening immediately.
Neptune Suggested by dragonjek
Equestria teleported into the Solar System just fine. Unfortunately, Xlestia missed. Rather than the third planet Earth, Equestria reappeared in the eighth planet, Neptune. As ponies need oxygen and heat, the -200 Degree Celsius hydrogen/helium atmosphere killed most ponies before the landmass sank into the middle of the gas giant and was crushed by pressure 100,000 times greater than Earth's atmosphere.
Batman Suggested by skyranger BIGSKY, Phalanx132, jm0914, dragonjek and Shadow Man
Xlestia watched the ocean with some concern. Sure, so far this world was the same as the one she had scryed, but she hadn't ruled for over a thousand years by trusting that things always went according to plan. Still, nothing had gone wrong so... far...
Her thoughts trailed off as the loud, offensive noise of a motorboat became audible. Soon, the watercraft in question came into view. Painted on it was... the symbol of a bat?
"Good evening! May we have directions to... oh, Your Highness! A pleasure to meet you!" Came the voice of the driver, a human male dressed... in a bat costume?!
"Who are you?" Xlestia demanded.
The man's brow furrowed. "I am Batman, your highness. And this is my trusty sidekick, Robin."
"A pleasure, your Highness." The boy in red tights interjected.
"Are you sure you have not heard of us, your Highness?" Batman seemed very intent on the answer.
"No, I have not. I know of all sorts of famous humans, and I do not know of you." Xlestia replied firmly.
Batman nodded. "Very well then. Princess, I must inform you that I will fight your evil plans until my dying breath!"
The sun princess blinked. Robin gasped. "But Batman! How could a unicorn princess be evil?"
"I'm an Alicorn..." Xlestia muttered.
"I don't know Robin. But her nefarious plots are as clear to me as this fine summer water."
Xlestia looked down. The sea was a very thick blue here.
"You see, she did not know who the two of us were. But she did know other famous humans. How could this be; what is different about us?" Batman continued. "What is different is that while we are indeed famous throughout all of fair America, it is the fame of word-of-mouth. No written records of us exist anywhere. So what kind of person knows of written records but not spoken knowledge?"
Robin crossed his arms in thought. "It would have to be someone who didn't talk to people to find information out... someone who could read but not speak? Ah ha, a remote viewer!"
Xlestia was getting more and more confused. Their conclusions were correct, but their logic was so tenuous.
"And what kind of person uses such a method of observation? A spy! Why do spies spy? To prepare for battle!" Batman concluded. "Thus her actions here on Earth can be none other than the first stages of world domination!"
"Holy pastel ponies Batman!"
"All right, that's enough!" Xlestia roared. Horn blazing, she gripped Batman in a TK field and held him tight. "There are no room for nutcases in my world! I'm sorry, 'Batman', but it seems like you just met an unfortunate boating accident."
"Robin!" Batman gasped. "Quickly! Formula 37!"
"Of course, Batman!" Robin grabbed a bottle and threw it at Xlestia with a "Throw!" sound effect almost visible.
The liquid inside splashed all over her, and Xlestia felt a burning all over so bad so didn't notice when her TK field failed.
"Argh! What... what is this?" She cried in pain.
"A liquid of my own invention." Batman replied, massaging his throat. "I thought something like this might happen, so I came prepared."
You didn't know you would be fighting me until you arrived! Xlestia thought angrily.
"You see, your magic is clearly being channeled though that horn of yours. And much like a Rhinoceros, your horn is made of compacted hair, or keratin. That is why I prepared Formula 37 - anti-keratin."
Xlestia whimpered as her horn, mane, tail, hooves and fur all began to dissolve and fall off. Even her feathers were affected.
"You won't hurt anyone like that." Batman finished, looking pitifully at the 'naked' alicorn.
And I would have got away with it too, Xlestia thought. if I had fought someone who made sense.