SF (canon species only; no humans) Ponies were magically and genetically engineered to be the perfect servitor race. They are powerful, adaptable, intelligent and completely under the control of their creators. A laboratory accident frees one such pony from her mental chains, but how can one mare save herself and the rest of her kind if she doesn't even know she's a slave?
This is not the Equestria you know and these are not your little ponies... not yet, anyway.

Tags: Adventure, Diamond Dogs, Gryphons, NO HUMANS (did I mention that already?)
Other tags: historical, mind control, Clarke's 3rd Law, excessive use of high energy physics, before they were famous.
Detailed review by PaulAsaran.
Cover art by endrome , used with permission.
Russian translation (partial -- 'unofficially' completed in the comments)
A Soot-Covered World, book 1.

When the Cutie Mark Crusaders dig up an ancient magical artifact, they unleash a spell on the town that allows everypony to see a "lie meter" floating above each other's heads. While attempting to solve the mystery, Twilight Sparkle has to analyze the tenuous balance between friendship and honesty. She doesn't like what she finds.

Based on a story idea conceived by theworstwriter
Written and posted with his permission

A romance story with Twilight and Trixie, based in a world where Twilight and Trixie are a couple, and get together in secret to go on dates.

Twilight becomes alicorn of the stars. This is sort of a problem, because Luna kind of already was alicorn of the stars. Oops!

TV Tropes

Hanna, the CEO of Hofvarpnir Studios, just won the contract to write the official My Little Pony MMO. Hanna has built an A.I. Princess Celestia and given her one basic drive: to satisfy everybody's values through friendship and ponies. Princess Celestia will satisfy your values through friendship and ponies, and it will be completely consensual.

Kuno, a young changeling, is left weak, bloodied, and bruised after the events of the Canterlot wedding. She is soon discovered by Warden, a Royal Guard. But for some reason, Warden shows her mercy. Instead of taking her into the custody of the Royal Guard, he becomes both her captor and her savior. But over time, Kuno will come to ask herself the hardest question she has ever pondered: Can a changeling feel love?

For almost a thousand years, the immortal Nightmare Moon has reigned over Equestria as Empress. To the average pony, Nightmare Moon is an overarching enigma whose rule is absolute and whose wrath is terrible. But to Twilight Sparkle, who as an orphan has never known her parents, the Empress is something else entirely. When the young unicorn is brought under the royal wing as an apprentice, she learns a great deal more about the supposed tyrant than she ever knew. But as Twilight is introduced to life with her new mentor, she is thrust into a world of political intrigue, conspiracies and secrets. Secrets that were never meant to be revealed. As allies and enemies plot against the very peace of the nation, will Twilight fall prey to the perils of her new position? Or will she prosper and find in Nightmare Moon the mother she never had?

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My eternal gratitude to my editor, LordOfTheWrongs, without whom Night's Favoured Child would not be the story it is today.

Now with added TVtropes!

Luna discovers something hidden from Celestia for twenty years, now, with emotions running high, she enlists the help of the Mane 6... but what seemed to be a single mystery, soon reveals a darker and much more complicated plot that will soon decide the fate of Equestria.

Check out the Empty Room TV Tropes Page (Warning: Spoilers!)

Revised chapters edits and proofing (with many thanks) by: lammy, Trevor, Nick Nack, Fifth Alicorn

It has been ten years since the defeat of the Changling Queen, and Equestria is again at peace. Twilight Sparkle and the Elements of Harmony have reached their prime, and have begun to settle down. Love and life abound in Ponyville, especially for Twilight's friends, but she has nopony to call her own.

There is a reason.

A series of events lead to a horrible revelation, and Twilight is thrust deep into a terrifying web of secrets and lies. Her peace is over.

This time, however, she won't be fighting monsters or madponies. What she faces will be much, much worse.

Act One of Two.

Act Two is here.

Currently undergoing editing by the wonderful Mstar!

On a dark night filled with even darker magics, an insane cult attempts to give Nightmare Moon a body and life of her own, utterly separate from Princess Luna. But, when the spell is interrupted by Celestia, something unexpected is created.

Now, a little black alicorn filly named Nyx finds herself living under the care of Twilight Sparkle, only to be haunted by memories and emotions from her past. Is she Nightmare Moon reborn or is she simply a doppelganger with a soul and mind of her own? Can Twilight Sparkle protect Nyx from those who refuse to see past her dragon eyes and black coat?

Or, will Nyx be forced to inherit the sins that may not even be hers and become the greatest evil Equestria has ever known?