Terrato

Terrato is Princess Celestia and Princess Luna's brother, kept hidden from the Heartland because Celestia wishes to keep the Heartland ponies in a state of blissful ignorance from the wars Equestria is having with its neighbors. He commands the Equestrian Legion, the army responsible for fighting in these wars.

History
He was originally part of the Eternal Herd until Lexarius lost control and committed the 2nd rebellion. He along with his sisters were sent to Equestria to defeat him, and they succeeded by using the Elements of Harmony to turn him into stone. However, afterwards the three of them decided to take matters into their own hooves and instead of returning to the Eternal Herd, they stayed and ruled over Equestria themselves, committing the 3rd rebellion. Celestia decided she wanted the Heartland of Equestria to be blissfully ignorant of the ensuing wars between Equestria and its neighbors, so she constructed a magical barrier around the Heartland but not before inviting several prominent families of ponies into the Heartland. Prince Terrato remained outside the barrier to command the Equestrian Legion, and was forgotten about by the Heartland.

During the Nightmare Moon incident, he was tasked by the King and Queen to kill Luna, but Celestia decided instead to use the Elements of Harmony to banish her to the moon, committing the 5th rebellion. He is initially distrustful of Luna after her return because of this.

Sometime after the Nightmare Moon incident but before the events of The Breaking Point, he gains a protege in the form of a unicorn mare named Black Rose and ends up falling in love with her. Due to their romantic relationship, he ends up teaching her more than she was supposed to know to the point where she becomes almost as powerful as an alicorn, committing the 6th rebellion. She then decides to assassinate Celestia in order to make Terrato the sole ruler of Equestria, but Terrato was forced to kill her.