Ybgir

Ybgir is an interdimensional demon who appears in "Jinx", who is also the main antagonist for the episode after Rigby does a forbidden jinx "cheat code" in the bathroom (fogging up the mirror, locking the door, turning on the hot water in the sinks, writing his name in the mirror, then saying it three times) to get rid of his jinxing. Ybgir seems to be the exact opposite of Rigby, as he is taller and more muscular. But it could be that it has no true form, only transforming into the being that uses the forbidden jinx cheat code, such as Rigby.

Ybgir has an ability similar to that of werewolves, as it scratches its victims to turn them into other negative demons, although his power had no effect other than amnesia upon dying/returning to the mirror dimension. It is also suggested that his name is Ybgir, because they must say the monster's name three times to send it back where it came from, but it may only be that way for Ybgir and may be the reverse name of whoever the jinx cheater is, not to be confused with the Ybgir of Dizzy World in "Dizzy".

Appearance
A negative monster is a monster from another dimension from behind the bathroom mirror. In Jinx, it looks like a monstrous, muscular Rigby, though this could not be its true form as it will become whatever species the person trying to break the jinx is. Either way, a negative monster has purple eyes, black skin/fur, and white outlines, giving the appeal that the monster is in a negative art style.

Trivia

 * How Rigby "unjinxes" himself is similar to the Bloody Mary myth. The myth goes: if you lock yourself (or more people) in the bathroom, turn the lights off, look into a mirror and say "Bloody Mary" (in his case "Rigby") three times, then you would see Bloody Mary. This is similar to what Rigby did since he did all of the things you do for the Bloody Mary myth, though Rigby adds two more things (the water faucets and writing his name).
 * Muscle Man told Rigby to do so because he thought it was a joke, so he told him to say, "Rigby, Rigby, Rigby" to a foggy bathroom mirror 3 times, unknowingly that it would send Ybgir out to the world.
 * His name also appears to be Ybgir, not to be confused with Ybgir from the episode "Dizzy".
 * His appearance is similar to that of the Loboan from Ben 10, but his power to turn people into strange purple creatures is more like that of the material the alien mummy used.
 * When he tries to attack Muscle Man and Hi Five Ghost, it only affects Muscle Man, considering the fact that HFG is already dead.
 * When people are scratched by Ybgir, it seems that it gives them a beard.
 * Ybgir is an evil nega-clone of Rigby with his name in reverse.
 * Every time Ybgir scratches someone, his muscles appear to grow.
 * It seems that Ybgir will also attack someone if they talk, as if unleashing him causes everyone to be jinxed.
 * When Skips becomes un-zombitized by Ybgir, he remembers everything. When Rigby and Mordecai ask how, he says it is because his "Mind is a steel trap".
 * Ygbir is listed as "Negative Rigby" in the credits.
 * Ygbir was not revived by Garrett Bobby Ferguson Jr. in Exit 9B.