In legends, that Yōkai is represented as a giant tortoise, a snake coiling around it. They are rare, though, so there is little known about them.
Yōkai of this type may be calm and patient like the tortoise, or sneaky and mysterious like the snake. Not much is known about them.
The Black Tortoise is one of the Four Symbols of the Chinese constellations. The word for "tortoise" was taboo; and the entire entity is not just the tortoise itself, but both the tortoise and the snake. It is sometimes called the Black Warrior of the North (北方玄武, Běi Fāng Xuán Wǔ), and is known as Genbu in Japan and Hyeonmu in Korea. It represents the north and the winter season. Although its name in Chinese, Xuánwǔ, is often translated as Black Tortoise in English, it is usually depicted as both a tortoise and a snake, specifically with the snake coiling around the tortoise. It is said to represent the Water element.
In ancient China, the tortoise and the snake were thought to be spiritual creatures symbolising longevity. During the Han Dynasty, people often wore jade pendants that were in the shape of tortoises. Because of ancient Chinese influence on Japan, honorific titles and badges in Japan often referred to the tortoise or images of tortoises.
The tortoise is a symbol of long life and happiness. When it becomes one thousand years old, it is able to speak the language of humans. It is also said to be able to foretell the future.
The tortoise is also regarded as an immortal creature. As there are no male tortoise - as the ancient believed - the female had to mate with a snake. Thus the image of the tortoise embracing a snake.