The kamaitachi (鎌鼬) is a yōkai, a supernatural creature of Japanese mythology, traditionally associated with the wind and widespread in various parts of Japan, especially mountainous and, indeed, windy.
There are many versions of this spirit, partly different in appearance and characteristics depending on the area of sighting, but in general it is a very fast being looking like a weasel (traditionally considered an evil animal),
which moves riding on gusts of wind and is equipped with razor-sharp claws with which it injures the legs of unfortunate passers-by and then immediately disappears.
The action is so fast that often the victims do not even notice the attack, also because, another peculiar characteristic of the kamaitachi, the wounds inflicted do not cause pain but only bleeding, sometimes copious.
According to some versions, instead, the exact opposite would happen, that is the wounds would not bleed almost at all but would cause great pain and in some cases would be fatal.
The most famous version of the kamaitachi originates in the mountains of the Mino and Hida regions (today united in the prefecture of Gifu), where it seems that it appeared as a trio of weasels of which the first one made the victim stumble, the second one cut the skin of the legs and the third one cured the wound with a medicine able to eliminate the pain.
This interpretation seems to be attributable to Toriyama Sekien, who was probably the first to associate the apparition to the weasel;
he performed, in fact, a typical play on words, slightly altering one of the most popular names of the creature, kamaetachi (構え太刀 "tachi attack"), to transform it into kamaitachi (鎌鼬 "weasel with scythes").
In Niigata prefecture, however, the kamaitachi was a single spirit but much more aggressive, so that its victims could not get rid of it.
The image of weasels armed with sharp razors has recently made its way into several manga and anime, which indirectly pay homage to the mythological creature.
Here's a short list of appearances: