Humans.

Truth or myth? Ancient lost civilization or just an old pony's tale?

No pony has ever seen one, and most haven't even heard of them. But Lyra knows that these wonderful creatures are more than just old legends, and she's going to find out... and possibly drive her roommate insane in the process.

Alone and far from home, Luna has sought out the changelings. Completely in their power Luna is entirely at the whim and mercy of the Queen of changelings, but what has prompted this strange encounter?

Part 1: Lyra's Pyro Predicament
Part 2: Twilight's Demoman Devastation
Part 3: King Sombra's Robotic Retaliation

After receiving a strange book from Pinkie known as "The Bombinomicon", Twilight wakes up one morning to find that the mystical tome has been stolen, by...Twilight?

Things quickly unravel as RED Team's Tavish DeGroot (Demoman) and Spy find themselves not only trying to get back home, but also fighting for their lives as villains from the Team Fortress 2 universe combine forces with some of the most dangerous foes Equestria's ever seen!

Part 1: Lyra's Pyro Predicament
Part 2: Twilight's Demoman Devastation
Part 3: King Sombra's Robotic Retaliation

When a mishap causes two para-dimensional weapons to colide with one another, RED Team's Pyro is sent flying from the Team Fortress 2 universe into the alternate universe of My Little Pony, where he is almost instantly discovered by Lyra, who shelters him in her home.

Having him hidden in her small townhouse isn't easy; keeping him from setting things on fire, teaching him that not every cart that passes by is cause for alarm, and that blue ponies are not something to be suspicious of.

However, with Nightmare Night soon approaching, Princess Luna feels that there may be something more sinister riding on the coat tails of Pyro's grand entrance; the first human to ever set foot in Equestria some thousand years ago...

REWRITTEN IN 2018! WOO!
Equestria Daily
Here's the music a friend made for the book based on how he felt when reading it.
Da semi-sequel be here.

Ignore the first story with his name. It is just what passed through his mind one day. This is his true appearance.

The Psychopath is a stallion with an unknown relation to the princesses, and yet he is held within the lowest levels of the Canterlot castle dungeons. He is completely chained up in his dark and dry cell, only ever thinking about his next prank and humming tunes that come from nowhere. His existence has been kept secret for centuries, and only the council and a few select guards know of him. Now, another mare will know of his existence. Twilight Sparkle will free him inadvertently, causing the psychotic pony with a strange appearance to float about Equestria, causing mayhem and destruction in the form of "pranks". How will the student deal with somepony who is apparently not trying to cause harm but does so anyway?

The image was made by Stalin the Stallion . Thank him for giving about thirty minutes of his life to draw that. (How did he even do that in half an hour?)
Proof-reader and aid for rewrite: Majora and masks. (Obsolete)
Proofreader: Twilight Scratch. Spank you very much for helping me. (Obsolete due to rewrite but they did help in the past)