What is "an-doughnut"

An-doughnut (あんドーナツ, an-dōnatsu) is a Japanese doughnut filled with red bean paste. It is a confection created in Japan, along with anpan, Jam pan, cream pan, curry bread, and many others. It is unknown when an-doughnut was created in Japan. However, Mister Donut in Japan added an-doughnuts to its menu in December 1983.