According to beliefs from northeastern Japan, a long-lived bat transforms into a Nobusuma, which then, after many more years, transforms into a yamachichi. These yokai resemble monkeys with pointed mouths and sucking lips.

Yamachichi live deep in the mountains and pay visit to houses late at night. They steal the breath from their sleeping human victims, sucking it out of their mouths with their pointed lips. After sucking away all of their victim’s sleeping breath, the yamachichi taps its victim on the chest, and then flees into the night. A human who has had his or breath stolen this way will die the next day. However, if a yamachichi should be caught in the act of stealing someone’s breath (either by the victim or by another witness), it will flee, and their victim will actually have their life span greatly increased instead.

The name yamachichi only appears in Ehon Hyakumonogatari, an Edo period yokai bestiary, and thus very little is known about them. The characters used to write the name literally mean "mountainous region" and "breast" or "milk", but these are most likely ateji — characters assigned phonetically without regard to the original meaning of the word. The original meaning of the name is mysterious and the only explanation given is that they are called yamachichi because they live hidden away in the mountains.

Because they are very similar in shape to satori, yamachichi are often confused with this yokai, and have picked up the alternative name satorikai.