It is said that because there is a legend that, at the time of the Bunroku-Keicho War, Kiyomasa KATO loaded wet soybeans onto the back of a horse which fermented because of high body heat of the horse, and became natto.
これは、加藤清正が文禄・慶長の役の際濡れた大豆を馬に積んでいたのが馬の高い体温で発酵し納豆になったとの言い伝えがあるからだとされる。