Nightmare Bonnie is one of the secondary antagonists of Five Nights at Freddy's 4. He is the nightmare form of Bonnie. He also represents one of the Big Brother's friends.
Nightmare Bonnie's form has been massively changed. He seems completely damaged like his appearance in Five Nights at Freddy's 2, with more vicious look. He has a purple glow on his right eye. His jaw is huge and filled with many sharp teeth. His finger also ends with sharp claws.
Five Nights at Freddy's 4
His role parallels that of Nightmare Chica, coming in from the west hall of the house. Unlike Chica, however, upon entering the room to attack, he himself attacks the child. He is also the more frequent attacker out of the two. The child has to hold the left door shut to prevent him from getting in. If the child flashes the light at him when he is standing right outside the door, he will attack him, and if he turns his back on the door to look at the bed, he will enter the room and attack. The only way to tell he is at the door is to listen for his breathing. Holding the door will cause him to walk away, being unable to enter.