Momonjii (in Japanese: 百々爺) is one of two different transformations that extremely old Nobusuma may become in ancient Japanese folklore. As such, they are an advanced form of yokai, and are depicted as an extremely old, feral humans with thick hair and monstrous traits, betraying their ancient origins as a bat-spirit.
The Momonjii has lost most of his vampiric nature, and no longer stalks travelers to steal their lantern fire, nor does he hunt down small animals to drink their blood. In fact, what the Momonjii eats (if anything) is a mystery, and he instead haunts lonely roads or out-of-the-way places.
If encountered, the Momonjii will cause people to grow very sick. And while not being a lethal yokai, he has no qualms about beating any he comes across, especially misbehaving children, or even children who simply cry (making him a form of bogeyman).
While a rather unpleasant yokai, he is not nearly as dangerous as the other forms of the Nobusuma, though his ability to cause sickness does make contact with him hazardous to mortals.