The royal court of Canterlot welcomes its newest member with open forelegs: the human philosopher Voltaire, from Earth.

On Earth, Voltaire is known as the greatest enemy of tyranny in the history of literature, and a legend in his own mind.

On Equestria, the court betting pool is giving him a week before the Princess banishes him to the Moon.

Note: This story doesn't have sequels per se, but there are a couple of short follow-ups set in the present day ("Parade Coverage" and "Secret Histories"), and a prequel of sorts in "At the Inn of the Prancing Pony".

TV Tropes page.

An aspiring rationalist gets punted into Equestria - and instead of being turned into a cool griffon, or powerful dragon, or even a standard pony... discovers he is now a milk-cow, part of the herd.

Part of the Chess Game of the Gods crossover.

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.

After the Changeling invasion, Celestia and Luna make the decision to have Twilight investigate the Elements of Harmony and solve the mystery of their power.

A thousand years later, Rainbow Dash awakens in a world she does not recognize. She has no idea how or why she came to be there. She does not know the language, the geography or any of the rules of this new time. She soon finds allies in Star Fall, a scholarly pegasus whose Talent is Magic, and Astrid, Star Fall's Griffin guardian. Together they set out to discover why Dash has been sent to the future, and how she might return to when she belongs.

War threatens the nations, a Nightmare stalks the shadows, and Rainbow Dash's arrival has turned her into a wild card in a deadly game that pits the law on both sides of a broken Equestria against a mysterious criminal figure, Max Cash. Yet the stakes are higher than anyone knows, for in her research Twilight Sparkle discovered a dark secret about the Elements, something that will shake the foundations of the world.

Cover image by SierraEx
Has a TvTropes page

A Diamond Dog king, feeling bold after gathering an army of 100,000 dogs, decides the best way to flex his new muscles would be to invade Equestria for past perceived slights. As such, he sends a letter to Princess Celestia with a declaration of war.

It goes about as well as you'd expect.