Larry

Shadowfright, sometimes called Larry by his minions, is the main antagonist in the second arc of the comic series (dubbed the Nightmare Arc). He is the leader of the Nightmare Forces, a race of dark shadowy creatures that create nightmares. He only appeared in the comics.

Shadowfright has the appearance of dark smoke, bigger and darker than any of his minions. He also gets angry every time one of his minions calls him Larry.

Depiction in the comics
In Issue 5, though Shadowfright himself is not mentioned, it is revealed that when Princess Luna was corrupted/possessed as Nightmare Moon, Shadowfright and the others would tell her what to do to get ponies to fear her, giving her their energy and hate. She became queen of the Nightmare Forces. It is possible that Shadowfright was the one who turned her into Nightmare Moon by magnifying her jealousy and bitterness, but this is not much elaborated on. When Luna was restored by the Elements of Harmony, she thought Shadowfright and his army would wither and die without their queen, but they instead went to the moon, where they would be granted one more chance in the cycle of the new moon to claim a kingdom of their own, as Nightmare Moon had promised them.

After giving her a nightmare about her friends rejecting her for a pony resembling Mabel from Gravity Falls, the Nightmare Forces kidnap Rarity and take her to the moon on the Nightmare Dreamscape. Shadowfright appears in person in Issue 6, and transforms Rarity into Nightmare Rarity to be their new queen. The Nightmare Forces use nightmares on Twilight Sparkle, Rainbow Dash, Pinkie Pie, Fluttershy, and Applejack, but Luna and Spike snap them out of it. Shadowfright tries to convince Luna to become Nightmare Moon again, but she declines, so he reveals Nightmare Rarity to everypony, saying that with Rarity possessed, the Elements of Harmony are useless and Ponyville is defenseless.

He will most likely appear in Issues 7 and 8.