For its pure strangeness.
The Makura-gaeshi (”pillow-mover”) is a soul-stealing prankster known for moving pillows around while people sleep. The creature is invisible to adults and can only be seen by children. Anatomical features include an organ for storing souls stolen from children, another for converting the souls to energy and supplying it to the rest of the body, and a pouch containing magical sand that puts people to sleep when it gets in the eyes. In addition, the monster has two brains — one for devising pranks, and one for creating rainbow-colored light that it emits through its eyes.
And I like the willow witch:

Yanagi-baba (”willow witch”) is the spirit of 1,000-year-old willow tree. Anatomical features include long, green hair resembling leafy willow branches, wrinkled bark-like skin, a stomach that supplies nourishment directly to the tree roots, a sac for storing tree sap, and a cane cut from the wood of the old tree. Although Yanagi-baba is relatively harmless, she is known to harass passersby by snatching umbrellas into her hair, blowing fog out through her nose, and spitting tree sap.
For others, visit the Pink Tentacle.
Hat tip Mo Costandi. Mo’s hanging out in some strange places.