Tantabus

The Tantabus is the main antagonist of the My Little Pony: Friendship is Magic semi-season finale Do Princesses Dream of Magic Sheep? It is a dark and parasitic magical force that turns dreams into nightmares.

Role
The Tantabus was a manifestation of darkness created by Princess Luna to punish herself for the things she did as Nightmare Moon. It resided in her dreams and gave her nightmares, such as her turning her back into Nightmare Moon. But then in one such nightmare, the Mane 6 arrived and attempted to turn her back with Rainbow Power. Having gained enough power from Luna's nightmares, and learned of the existence of the Mane 6, the now self-aware Tantabus escaped into the dreams of the Mane 6, giving them nightmares.

The Mane 6 contacted Luna, who entered their dreams as they went back to sleep to find and defeat the Tantabus. One by one, Luna chased it through the dreams of the Mane 6: In Rarity's dream, it turned her dresses into monsters; in Pinkie Pie's dream, it turned cakes into monsters; in Fluttershy's dream, it turned Angel into a monster; in Applejack's dream, it turned all the apples to the state they are in after being sucked by Vampire Fruit Bats; in Rainbow Dash's dream, it turned her favorite dream of her fighting Changelings into a nightmare of "unbearably cute" flowers singing a song similar to Barney; and in Twilight's dream, it turned books into monsters that attacked her.

Luna was unsuccessful in catching the Tantabus, and having seen all the ponies of Ponyville in Pinkie's dream, the Tantabus was able to spread its influence to them. If it became powerful enough, it would be able to enter the real world and turn Equestria into a world of nightmares. Luna used her magic to make it so that the Mane 6 and all of Ponyville were in the same dream, so when the Tantabus arrived, they fought it off, able to do anything within the dream, however, they were unable to defeat it.

When Luna said the entire thing was because of her, the Tantabus got even bigger. She revealed to the Mane 6 that she created the Tantabus and the reason for which it was done. By this time the Tantabus had taken the form of a giant Nightmare Moon and was on its way through the portal; however, when Twilight convinced Luna to forgive herself and move on from the past, the Tantabus was absorbed back into Luna, vanishing for good.

Trivia

 * The Tantabus is the second villain to be created by a major villain (the first being the Plunderseeds who were created by Discord).