Nightmare Balloon Boy is the nightmarish version of Balloon Boy and is a major antagonist in Five Nights at Freddy's 4: Halloween Edition. He is one of the few new Nightmare animatronics alongside Nightmare Mangle and Nightmarionne. He's featured in his own minigame known as Fun with Balloon Boy, and thus functions similarly to Plushtrap.
Nightmare Balloon Boy appears similar to the original BB, except with a more nightmarish design. This includes large teeth inside of his large mouth (similar to Nightmare Chica), long claw-like fingers, and more sinister looking eyes. He has a red nose and cheeks instead of orange ones. He also has a red and purple striped hat and shirt, instead of his usual red and blue stripes. Interestingly, Nightmare Balloon Boy appears to be in a perfect and completely undamaged shape.
Nightmare Balloon Boy only exists in the Fun With Balloon Boy minigame, which functions almost exactly like Fun With Plushtrap.