They threaten the life of a Calcutec in Hard-Boiled Wonderland and the End of the World, and Akutagawa wrote a novella about these crafty creatures that always stink of fish and try to lure human beings into the depths of ponds to eat them.  
Because their victims are often found with swollen rectums, they are sometimes called "anus vampires."  One of them tried to trick a man on horseback into playing tug-of-war, but when the man jeopardized the liquid on top of this creature's bowl-shaped head, this creature panicked and offered to teach the man the lucrative skill of bone-setting.  Another tale describes Genta hanging one of them from a tree.  A sub-class of the Sujin, they smell blood from miles away, wear tortoise shells, and can be bribed to leave potential prey alone with offerings of inscribed cucumbers.  They threaten the life of a Calcutec in Hard-Boiled Wonderland and the End of the World, and Akutagawa wrote a novella about these crafty creatures that always stink of fish and try to lure human beings into the depths of ponds to eat them.  FTP, identify these Japanese demons with monkey-like features.
Because their victims are often found with swollen rectums, they are sometimes called "anus vampires."  
Kappa or Kawako or Karanbou or Enkou