Tantabus

In the MLP Episode "Do Princesses Dream of Magic Sheep?"
The Tantabus is a being created by Princess Luna from before the Interdimentional Revolution began. It was created to punish Luna for becoming the villain known as "Nightmare Moon". The being inevitably betrayed it's mistress and went on to attack innocents, especially the Mane 6, turning their dreams into nightmares, and bring upon the MLP Universe a new reign of terror. It was then defeated in a battle within a dream realm designed to destroy the Tantabus.

After the MLP Episode/Fluttershy's depression
However, the creature would thrive through the subconscious fears of one the Mane 6, specifically Fluttershy. Later, Fluttershy would meet a Jedi Knight named SuperFolder Ghostbuster, in disguise as a simple pony. The two would develop feelings for one another over time, but Ghostbuster would eventually be arrested, and thrown into carbonite. This would leave Fluttershy in a state of endless depression and fear, which would have the Tantabus manipulate her through multiple nightmares of Ghostbuster dying in front of Fluttershy's eyes, each one worse than the last.

This would worry her friends, and get the attention of Luna, who discovered the Tantabus' survival. Fluttershy and Luna confronted the beast once more, but weren't strong enough their second time around, as Fluttershy would be too tired and depressed to fight the Tantabus, due to the visions it gave her. The Tantabus, however, would be tricked and defeated after discovering Ghostbuster's survival. The Tantabus would try to fight Ghostbuster with his worst nightmare, only for it to backfire, with Ghostbuster claiming to be his own worst nightmare.

Jedi Nightmares
The Tantabus would once again be assumed to be destroyed, which would again be proven wrong. After the fight, the Tantabus would move from Fluttershy's subconscious into Ghostbuster's subconscious, in a last effort for survival, knowing Ghostbuster's worst fear is himself. Whenever Ghostbuster slept, he only dreamed of fighting his darker self, only for it to actually be the Tantabus in disguise. Ghostbuster knew all along, but didn't care. Ghostbuster instead used it to his advantage, and trained subconsiously through the Force, using what he learned in his dreams from the Tantabus in actual combat against the Royal Guards, Twilight Sparkle, Princess Celestia, or worse. The Tantabus realized later on that it was once again tricked into becoming Ghostbuster's prisoner.

Fresh Meat
After the war, Ghostbuster would have a son named Firefly, who would be a very potential Jedi Knight. However, Firefly would be clouded by many fears, such as becoming a carbon copy of his father, Princess Celestia, and even some terrible memes. This would turn out to be the Tantabus moving from Ghostbuster's subconscious and into Firefly's subconscious. Seeing as Firefly wasn't a power Jedi yet like his father, the boy would be tortured by the creature for many, many years without notice.