Nyubachibo (literally, mortar [a bowl-shaped vessel] bonze) is one of Japanese yokai (ghosts, spirits and monsters) that was transmuted from a copper disk (cymbal) used as a musical instrument for theatrical performance. It is considered to be a kind of tsukumogami (a type of Japanese spirits that originate in items or artifacts that have reached their 100th birthday and become alive).
Sekien TORIYAMA's 'Gazu Hyakki Tsurezure Bukuro' (The Illustrated Bag of One Hundred Random Demons; the term 'hyakki' in its title is a pun on the usual hyakki, replacing the character for demon which is written as "鬼" in Japanese with a character for vessel written as "器," and sure enough, most of the yokais shown in this book are tsukumogami) portrays the Nyubachibo in a human-like figure wearing a copper disk (or cymbal) on the head, along with a Hyotan-kozo (a calabash or bottle-gourd boy) which is a specter of bottle gourds. Since a similar copper disk (or cymbal) yokai was depicted in "Hyakki Yagyo Emaki" (The 'Night Parade of One Hundred Demons' Picture Scroll) in the Muromachi period, some believe that Nyubachibo was modeled on it.
One theory has it that the Nyubachibo scares people by making a loud sound.