A bakeitachi (化け鼬, ばけいたち, "Monster weasel") is a type of yōkai that resembles or was originally a weasel. These demons are fairly common and often live around humans; hiding in their dwellings or nearby caves. They are often responsible for possessions which can cause illness. They are recurring foes for exorcists such as Miroku and Madame Exorcist.
The baketachi are part of a rather large and notable subsection of yōkai that include kitsune (bakegitsune), tanuki (bakedanuki), and bakeneko. The "bake" (化け) prefix in the names of these yōkai indicates their "monster" status, and differentiates them from ordinary foxes, weasels, raccoon dogs, and cats. All four are known for their mischievous nature and powerful magic, most prominent among which is the ability to shapeshift. These powers are obtained after living to a sufficiently long lifespan. Specifically, according to some traditions, a weasel becomes a bakeitachi after living a hundred years.
Although bakeitachi appear less frequently in Japanese folklore and pop culture (including manga and anime) than their counterparts, many regions' folklore holds them to be more powerful than foxes or raccoon dogs. According to tradition, a kitsune has seven disguises, a tanuki has eight, and a weasel has nine, indicating the weasel's apparent superiority. In the series, these numbers are reflected in the names of Shippō (a fox) and Hachi (a raccoon dog); the "shi" (七) in Shippō means seven, and "hachi" (八) means eight.
Unlike kitsune which steal energy and tempt men into romantic relationships, and unlike tanuki which merely wish to trick humans into looking stupid, baketachi seem to have more uniquely destructive motives, and large groups of them are known to set buildings, such as houses or temples, on fire. To hear the voice of a weasel was said to be an ominous sign of either great misfortune, or conversely, prosperity.
There was also a certain subtype of bakeitachi, known as kamaitachi (鎌鼬), that was said to travel in a whirlwind (similar to Kōga) and slash unsuspecting passersby with its long scythe-like claws. There was apparently no rhyme or reason to their actions, and they were especially prominent in mountain passes and in the snowy northern regions of Japan.
Despite being depicted in folklore as the more powerful of the shapeshifting yōkai, weasel demons in InuYasha tend to be very weak, and are never shown to use leaves to transform, in the manner that Shippō or Hachi do.
An ongoing joke in the series is the common misattribution of the aforementioned group of shapeshifting yōkai as raccoon dogs. Shippō often is mistaken for a raccoon dog, to which he takes great offense. Kanta, an otter demon, and a giant weasel demon are all mistaken for raccoon dogs. This recurring mistake on the part of characters in the series is an apparent reference to the fact that "tanuki" in the past had no uniform meaning, and was variously applied to weasels, cats, foxes and raccoon dogs. To add to the confusion, both weasels and raccoon dogs were often grouped under the term "mujina" (狢, badger); an actual badger demon named Mujina is featured in the series.