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 an argument with Luna, Celestia realizes that she is the only creature in Equestria who cannot sleep in. She enlists Twilight to help, and the two of them come up with a spell that will allow her to spend the night on a different world. Unfortunately, all their research failed to reveal that the world they sought was populated. Now, the Princess must decide whether to continue with contact, or destroy the only non-Equestrian witness to their existence.

Cover art is called "Sleeping Celestia" by Briskby

This story is a sequel to Celestia Sleeps In

After the accidental discovery of humans, Celestia tasked Lyra with learning about Dale's culture and Dale's language. Two meetings later, a misunderstanding caused Dale and a Coast Guard woman to be inadvertently transported to Equestria. With the two humans hospitalized and communication limited at best, the Equestrians just want to heal them, while back on earth the Coast Guard wants nothing more than to determine what happened to one of their sailors.

Twilight Sparkle wakes up one morning to a harsh dose of unreality. She is stranded, without her friends, on an island which matches no known locale on Equestria. Surrounded and harried on all sides by the alien and the dangerous, she will find her mettle sorely tested, and find perhaps that the best friends you will ever have in your life are the ones you make when your back is to the wall...

Thanks to CedricBale and noobie56xx for tracking down the name of the artist so I could give them credit.

Abderus was the first human to travel at superluminal speeds. His departure from Earth didn't go to plan, though, and now he finds himself marooned on an alien planet.

The star system Omega Centauri was just another oddity on a map to scientists in the not too distant future. However when they found the star was orbiting an earth-sized, earth-like planet instead of a black hole as its motion had suggested, a mission was scrambled to investigate this most unusual of celestial behaviors.

Hamstrung by politics, and nearly crippled before it began, the 'Lone Ranger' mission was reduced to just one crew member and left to his own devices.

These are the logs of Arrow 18 and its lone commander. This information is classified TOP SECRET by the Global Space Agency.

Do NOT tell the princess.

The Princesses find an alien creature barely clinging to life at the scene of a terrifying crash. Can the Combined Power of the Elements of Harmony save it, or more importantly, should they save it? Will the arrival of this strange creature tear Equestria apart?

TV Tropes
https://tvtropes.org/pmwiki/pmwiki.php/FanFic/Article2

It occurred one peaceful night. Half of Equestria awoke to the sound like a double-shot thunderclap. A percussion heard in the skies and rattling windows from Apploosa, to Canterlot, to Ponyville. Nopony knew what it was or what it heralded for the world as it drew a line across the nation.

For Twilight Sparkle, this isolated incident might have faded from memory after a time, if not for the strange rumors of a creature sitting on the hill on the outskirts of town.

These are her thoughts given written form. These are Sparkle's Notes.

Report EVERYTHING to the Princess.

King Sombra’s dark influence was obliterated by the Crystal Heart, and the villain is well and truly dead. But when he attempts to steal the afterlife of an innocent victim, a human spirit is left hurtling towards Equestria, where this unfortunate soul is revived into the only vessel available: the reconstituted body of King Sombra himself.

Thus, a cheerfully oblivious alien is crammed into the form of the most evil and malicious being Equestria has ever known. Not that Shining Armor and Cadence know this, of course. As far as they are concerned, the King is back and he’s lost his fricking mind.

Click here for the TV Tropes page!

When Apollo 11 lands on the moon, Neil Armstrong utters his famous lines as his boots land in the lunar regolith, but when he turns around he finds something no one was expecting or had ever planned for.
There's an alien on the moon. And it's blue, four-legged, and staring at him.

Sequel here.