According to Japanese belief, many animals, like birds and spiders, also develop into yokai when they reach a certain age. Japanese weasels, known as itachi, are seen as disconcerting animals and bringers of ill omens for the particular brand of magic that yokai weasels perform. Like most animals-turned-yokai, they possess shape-shifting abilities in addition to a number of magical powers.

In the old days, weasels were believed to trasform into ten (martens) or mujina (badgers or tanuki, depending on the region) after reaching a very old age. Additionally, the names ten and itachi were often used interchangeably. As a result, there is often a lot of confusion over which animal is specifically being referred to in many stories.

Itachi are tricksters and pranksters, but generally shy away from interaction with humans when they can. As a result, they are more mistrusted and disliked than most animals. Though they can transform, they prefer to use other kinds of magic, usually with unfortunate results for their targets. When an itachi is seen standing on its hind legs, it is said to be bewitching a human — perhaps hypnotizing them into leaving food out, or performing some other task for the weasel’s benefit. Itachi are said to be particularly dangerous in groups. When they gather together at night, they have the power to summon fire, climbing up onto each other’s shoulders and creating huge columns of fire which erupt into whirlwinds. These are frequently blamed for starting conflagrations which can burn down entire towns. In central Japan, the kama-itachi is another common and dangerous form. Their calls are also considered to be ill omens, for after the yelping cries of a group of itachi is heard, misfortune and despair always follows. For this reason, they are seen not only as dangerous yokai themselves, but as harbingers of greater evil.

Itachi are often considered to be the most skilled shape-changing animals of all, possessing more alternative forms than any other shape-changer. An old phrase about animal yokai goes, "Kitsune nana-bake, tanuki hachi-bake, ten ku-bake" — foxes seven forms, tanuki eight forms, martens nine forms. When an itachi changes its shape, it usually adopts the form of a young priest boy dressed in clothes that are too big for him. This form is used chiefly to acquire alcohol, which the weasels cannot brew themselves. Itachi also frequently adopt the forms of other yokai in order to scare humans. One of their favorites is the ō-nyūdō a colossal, bald-headed giant who terrorizes villages, destroys houses, devours livestock and sometimes even eats people.