|Other Names||The Oni, Blue Demon, Blue Troll|
|Location/Habitat||Abandoned Mansion, Tokyo, Japan|
|Language||Unknown, likely Japanese|
Aside from the Ao Oni and those it transforms, there are a number of mutant Onis. Each resembles a different version of the original and are locked in a cage in what is known as the Oni Room.
- Blockman Oni - This is a square-shaped oni that moves quicker than the regular Ao Oni. When it spots its victim, it pauses for a moment then charges at it with abnormal speed. In Version 6.23, it became an official enemy, replacing the Roach Oni of Version 5.2.
- Hunchback Oni - The Hunchback Oni is a normal oni, though its back is hunched over.
- Hulk Oni - This oni is a large, muscled oni that bangs the floor for no apparent reason. It also is sometimes referred to as "Squatto". Because of its size, it moves slowly in most cases. It is the largest of the onis. This Oni appeared as a secondary antagonist in Version 3, as well as many fan-made games.
- Deformed Oni - This oni resembles the regular Oni, though the oni itself is completely warped, hence the name.
- Blob Oni - Also known as the "Eye-see Oni", this oni is irregular. It looks like an amphorous mass with many eyes on it. The Blob Oni is relatively slow and sluggish.
- Encephalo Oni - This oni mutation is the head of the regular oni but lacks a mouth and has many tentacles. It bears resemblance to a jellyfish.
- Caterpillar Oni - This oni is also a sluggish type of Oni. It looks like a caterpillar with its head up. It has 5 eyes on each side and many, tiny legs.
- Flying Oni - This oni has spike-like protusions from its head and floats back and forth without any other movement.
- Clock Oni - The oni resembles a clock with thorns, as if exploding, or a stalactite. It levitates while staying affixed to its position, one of the thorns stretching and shrinking continously. It seems to be the flying oni looking straight ahead.
- Tall Oni - A regular oni, only taller. He's still just as dangerous
- Toothed Oni - This oni is similar to the original, but with its mouth open, revealing its teeth and tusks. It is also wide-eyed as opposed to its right eye being squinted slightly. In Version 5, this version appears in the character Takeshi's nightmare.
- Character Onis - A term given to the other Onis created when the Ao Oni kills one of the characters. All the characters, with the exception of the player character (Hiroshi) can become a Character Oni.
- Roach Oni - Appearing in Version 5.2 as a secondary antagonist, this Oni is similar to the Blockman. In appearance, it looks like a giant blue cockroach (hence the name) with an eye on the side of it's body.