Widespread battlefield use of mon seems to date to the Gempei War (1180-1185), where they were used on banners alongside Buddhist prayers, Shintō invocations, and heroic poetry. While it’s unclear if the two warring clans actually used the mon they were later associated with (butterfly for the Taira, gentian for the Minamoto), Heike Monogatari does contain clear references to mon on banners.(SH:10)

Taira butterfly and Minamoto gentian crests

By the Muromachi period (1336–1573), samurai commonly adopted mon to identify themselves. A compilation of mon used by provincial samurai from around 1460–1470 shows some interesting things. While most of the elements are recognizable from modern mon, there are some elements and variation that did not persist. Without a central authority, samurai were less constrained in terms of shape, arrangement, and combination of elements than post-standardization mon from the Edo period.(KJ:7)

Here is a sample from this compilation: a classic paulownia mon with the insertion of the character 安 or ‘peace’. These are both common elements, though the location of a character in the middle of the flower is an unusual arrangement that did not end up becoming standard.(KJ:7)