Screwball

Screwball is a curious anomaly of a pony. She is very closely associated with Discord, who was in some way or another directly responsible for her existence in The Return of Harmony part 2. In fan-fics, she has been portrayed as a devotee of the dragonequus and sometimes as his right-hand mare or harbinger. Alternatively, she has also been portrayed as an unfortunate victim, transformed into a being that is not wholly equine but not inherently bad enough for the Elements of Harmony to automatically fix.

Examples of Screwball's Portrayal
In School Daze, by Paleo Prints, Screwball is a strange but fundamentally good adolescent filly who owes her existence to Discord but adopts Cheerilee as a caretaker. She usually says the exact opposite of what she means, but can force herself to speak properly if she absolutely must be understood. Doing so seems to require dangerous amounts of mental strain however, as she tends to get a nosebleed in the process of reversing her tongue.

In the Screwball Trilogy, the eponymous pony is a product of pure chaos being channeled through an Obsessive-compulsive, rigourously structured mare named Button Stitch. Her "speech" is pure nonsense and sound effects and she engages in random and increasingly cruel pranks on Ponyville.

In A Head Full of Cotton Candy, by TheManWithTwoNames, Screwball is (at present) known as Savoir Faire, an advisor to the Canterlot Court who takes it upon herself, through some rationalization that it's expected of her, to guide Discord (who spontaneously came into being in reaction to high degree of conflcit within the kingdom and removed the King and Queen (Celestia and Luna's parents) from existence in the process) into assuming the role of King of Equestria. She's determined to keep Discord's child-like curiosity and predilection for experimenting at the very least channeled toward improving the lives of ponies across the kingdom. She frequently argues with herself whenever Discord takes matters into his own hands.