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)