The yamachichi (etymologically "mountainous region" and "breast" or "milk," a priori in the form of ateji) is a yōkai mentioned in Ehon hyaku monogatari.
According to legend, the yamachichi was originally a bat. After living for many years, it is said to have become a nobosuma (a kind of bat with powers) and then become a yamachichi.
This yōkai looks like a kind of monkey with a human face, sharp teeth, and sucking lips.
Legend of Yamachichi
According to legend, the yamachichi live in the mountains of northeastern Japan and descend late at night to enter nearby homes. The creature would approach its victim to suck in its snores, then punch its chest before fleeing.
Following its attack, the victim dies the next day, unless the creature is seen by witnesses. In this case, it will flee and the victim's life span will be extended.