She noticed the pony for the first time when she was young, not long after she'd received her cutie mark. He became her oldest friend.

Note: comments contain spoilers; please don't read them first!

Cover art by DaisyAzuras • Featured in the Royal Canterlot Library

Reading by Neighrator Pony • Spanish translation by Spaniard Kiwi

This story is a sequel to Our Illusion

Princess Rarity thought her life to be perfect. Leisurely days in the castle, friends all around, and the most lovingly lovely bodyguard she could ever ask for.

However, when tragedy strikes, she and her beloved must face the fact that sooner or later, whether they want to or not, all little princesses must one day become queens.

Part of the Bodyguard!AU Series, based on Earthsong9405's AU where Twilight is the bodyguard to a "Princess/Queen Rarity". Prior knowledge of the AU is HIGHLY recommended. // Cover art byidrawraritwi/lilfunkman.

Idol Hooves holds a special place in Equestria. He's probably the only exiled changeling to serve in the Royal Guard, or at least, that's what he expects, as exiles don't really broadcast that fact. However, it's well known that Princess Celestia loves all of her little ponies, and her guards perhaps more so, and that makes it one of the best ways for a changeling to discreetly feed.

But how does a changeling even get there, let alone get exiled in the first place? For that matter, why would it ever go to Canterlot? Set a number of years before the series proper, before Luna’s redemption, Idol details how he was exiled, the ponies he’s met, and the series of misadventures that led him to joining the Royal Guard, and should continue a good ways into the series, as something of an alternate perspective.

Soon, he’ll have to deal with a boisterous new princess to serve and a royal wedding in the works bringing a third to power. Will his old instincts bring this new life crashing down around him? What precipitates Chrysalis’s ill-fated assault on Canterlot? Is it really so wrong for a changeling to enjoy good craftsmanship to an obsessive degree?

New cover art provided by the fantastic Carnifex

And a TVTropes page here.

And an Ask Blog of questionable canonity!
Out of Idol Curiosity

Fanart by Norad2! Major thanks! Short For a Royal Guard

Art by Egophiliac of Idol and his Daring Duckie Please don't tell him what that brush is meant for.

Rating updated to (T), it's a bit darker than (E)
Comedy Tag added as well
And changed the character tags a bit.

This story is a sequel to Days of Wasp and Spider

Believed dead by their creators, the ponies that will become Celestia and Luna have broken their mental conditioning and escaped into the wilderness. Here they plan the revolution that will free all ponykind, knowing that it is only a matter of time before their ruse is discovered.

Back in their underground arcologies, the ponies' erstwhile masters deal with the consequences of the escape, all the while unaware that something lurking at the bottom of space-time wants them all dead.

A Soot-Covered World, book 2.
Canon species only/no humans.
This story has nothing to do with the Holocaust.
Art by InLucidReverie, used with permission.

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.

One winter's day, before Hearth's Warming Eve, Rarity falls backwards in time. To before all the Hearth's Warming Eves to come.

Written for Monochromatic for the 2017 Jinglemas collab. Cover art from the gallery of Plotcore. Proofread by themaskedferret.

Unhappy that her future as a member of the monarchy is denied by Celestia, Sunset Shimmer escapes through Starswirl's mirror into another world, planning to take it over to prove once and for all she was fit to rule. What she didn't expect—besides a species change—was to arrive in a world already conquered.

TvTropes Page (Just created! Needs Help!)

Art (Will contain spoilers)

Sunset Shimmer - Bloody Sunset

Spoilers Ch. 11+: Angel - Angel and Sunny
Spoilers Ch. 18+: Princess - Comic
Spoilers Ch. 121+: Alejandra Acevedo - Flying at the speed of death
Spoilers Ch. 161+: Apple Bloom - Bloom
Spoilers Ch. 180+: Food - Sunset's Party Dress
Spoilers Ch. 185+: Fugue - Fugue by Rametep
Spoilers Ch. 197+: Exhausted

(Art is always welcome!)

WARNING: CONTAINS SPOILERS FOR THE WHOLE STORY: TIMELINE