Ybgir

Negative Rigby is an inter-dimensional demon who appears in "Jinx", after Rigby does a forbidden "Jinx cheat code" in the bathroom (fogging up the mirror, locking the door, writing his name in the mirror, then saying it three times.) to get rid of his jinxing. Negative Rigby 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.

Negative Rigby 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 then amnesia upon dying/returning to the mirror dimension on High-Five Ghost. 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 Negative Rigby and may be reverse name of whoever the jinx cheater is, so not to be confused with the Ybgir of Dizzy World in a previous episode.

Appearance
Negative Rigby is a monster from another dimension from behind the bathroom mirror. In Jinx, it looks like a monstrous, muscular Rigby, though this cannot be its true form as it will become whatever species the person trying to break the Jinx the dark way. Either way, Negative Rigby 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 Blood Mary myth where if you turn the lights off, look into a mirror and say Bloody Mary three times, then you would see Bloody Mary and this is similar to what Rigby did because he turned the light off, looked into the bathroom mirror, and said his name three times and Negative Rigby came out of the mirror.