The Halloweenies inhabit the Nightmare Realm. All of them are monstrous in appearance, and some of them are doppelgängers of regular characters as this realm existed in Dib's imagination. They were dubbed "Halloweenies" by a confused and frightened Zim.
- Main Article: Nightmare Bitters
- Nightmare Bitters is the Nightmare version of Miss Bitters, and seemed to be the ultimate authority in the Nightmare Realm.
- Main Article: Nightmare Dib
- The Nightmare version of Dib. He was locked in the Nightmare Version of the Crazy House for Boys, and has a lobotomy scar on a head that's even bigger than that of his "real-world" counterpart. For most of his time onscreen, he remained in the darkness, only revealing his face once.
- Main Article: Nightmare Membrane
- Nightmare Membrane is the Nightmare version of Professor Membrane. Dib encountered him in the Nightmare Version of his own house, along with Nightmare Gaz.
- Main Article: Nightmare Gaz
- Nightmare Gaz is the Nightmare version of Gaz, and was seen in Dib's Nightmare House, whilst drinking "Dark Poop" soda.
Nightmare White Coats
- Main Article: Nightmare White Coats
- Nightmare White Coats were the Nightmare Versions of the White Coats, who work worked in the Crazy House for Boys.
- Main Article: Nightmare Nny
- Nightmare Nny is a Nightmare version of the real Johnny C and due to his appearance in the Nightmare realm, it can be implied that Dib and Nny met off-screen.
- Main Article: Keef
- Exactly the same as normal Keef. "He was already scary enough to begin with" - Jhonen Vasquez
Hummelflesh, Destroyer of Worlds
- Main Article: Hummelflesh, Destroyer of Worlds
- He seemed to be the leader of the Halloweenies (after Nightmare Bitters) until he failed in bringing Dib to Nightmare Bitters.
- Main Article: Yolk
- He was the Halloweenie who was sent by Nightmare Bitters to capture Dib after Hummelflesh failed.
- Main Articles: The Enigma
- He was the Halloweenie who attempted to reassure Nightmare Bitters when she felt stupid after saying "Let the head... thingy... start!"
Other Halloweenies are:
- The Admiral
- Bobby Joe Ray Billy Sue
- Bride of Mortïs
- Cage Monster
- Dancing Skeletons
- Gordon Peterson and Gordon Peterson Jr
- Mr. Pinchy
- Puncher of Heads
- Simon Rikkis
- Interestingly, despite the fact that all the Hallooweenies are in Dib's mind, none of them seemed to recognize that Zim is an alien.