WARNING: The following contains spoilers for One Piece Chapter #1020, "Robin vs. Black Maria," by Eiichiro Oda, Stephen Paul, and Vanessa Satone, available in English via Manga Plus and Viz Media.
The Wano arc of One Piece has become an homage to Japan's deep culture. From the character designs to the names to even the weapons, Wano Country is what Japan would look like in a fantastical world of pirates. Even certain Beast Pirates are designed with Japanese legends and culture in mind. In particular, Black Maria and her weapon Wanyudo.
In the manga, Black Maria has the head and torso of a beautiful woman, with the lower half of a spider courtesy of her Ancient Zoan-type Devil Fruit. This design is a callback to a monster of Japanese legend: Jorougumo. A shapeshifting yokai, Jorogumo takes on the form of a beautiful woman to lure men into her web before feeding on them. The kanji that makes up her name can be written to mean "entangling newlywed woman." Not only can she shapeshift, but some depictions even show her being able to control fire-breathing spiders.
Her weapon, Wanyudo, is a reference to another yokai that is essentially a face in the center of a flaming wagon wheel. Legends claim that the face is that of a daimyo, feudal lords subordinate to shogun. The reason for the daimyo's affliction is usually because of his cruel pastime of dragging people behind an oxcart, so as punishment, he now spends eternity either roaming country roads for victims or guarding the gates of Hell. His victims are usually criminals or corrupt priests... and the occasional baby. As Maria's weapon, its fire powers are fueled by a man-faced dog running in the center of the wheel.
The man-faced dog is called a Jinmenken in Japanese folklore. They tend to be sighted running at high speeds along highways and roads before running in front of a car, stopping, then turning to reveal their human face to the driver. As a result, they are said to be the cause of many car accidents. They are said to be capable of human speech, and usually, just yell at whoever sees them to "Leave me alone!" They are either the result of experimentation, or the spirits of people who were struck by a car while walking their dogs, though these origins vary from place to place and person to person. Jinmenken are usually played for comedic purposes, like in the anime Ghost Stories or Yokai Watch. Here though, it is nothing to laugh at, especially when it's being led by someone like Black Maria.
