According to Japanese beliefs, Kudan are prophetic creatures that take the form of baby cows with human faces. Very rarely, they are also said to take the reverse appearance: a cow’s face on a human body. They are born from cows, and their birth is often said to be an omen of some significant historical event. A kudan never lives for more than a few days.

Kudan are able to speak human languages from the day they are born. Immediately upon being born, a kudan gives one or more prophecies. The content of their prophecies varies. Some kudan have spoken of great harvests or terrible famines, some have foretold plagues and droughts, while others have predicted wars. The prediction of a kudan never fails to come true. Upon delivering its prophecy, a kudan immediately dies.

Kudan are a relatively recent yokai, having entered the public zeitgeist during the end of the Edo period. This was a period of great social and political upheaval. The fall of the shogunate and the return of imperial authority, combined with the rapid changes brought about by the opening of trade with the West were responsible for a lot of uncertainly and turmoil throughout Japan. During this time, stories of kudan being born popped up up in newspapers all across the country.

Kudan sightings continued through the end of the Edo period until after World War 2. Among some of the events supposedly predicted by kudan are the Russo-Japanese War and the Pacific War. Because of their uncanny ability to predict the future, the word of a kudan was viewed as absolute truth. During the Edo period, newspapers looking to add credibility to a story would include the words "kudan no gotoshi", or, "as if a kudan had said it" to their articles. This phrase remains in use in the Japanese language today as a way of assuring the reader of the truth of a story.

Because of its reputation for honesty, images of kudan were used as talismans for good luck, prosperity, and protection from sickness and disaster. Newspapers advised their readers to hang the printed images of kudan on their houses for protection and good fortune. Kudan were such popular yokai that their mummified remains were often carted around in traveling sideshows. These "kudan" could be made of stillborn deformed calves, or of different animal parts stitched together to create a chimera-like stuffed animal. Visitors paid a small fee to gawk at these specimens and hopefully receive some of their good luck. A few of these mummified remains survive in museums today.