Even before the golden period of Japanese heraldry, the beautiful butterfly pattern was a favorite amongst warlords and their samurai, from as early as the Nara period (710 AD - 786 AD)
Displayed as crest or emblem on their armor, it appears to have been favored for its apparent delicate nature and elegant symmetry, achieved through the evolution from lowly caterpillar to noble insect.
Samurai warriors of the ill-fated Heike (also know as Taira or Heishi) clan -Japan rulers from 794 AD until 1185 AD- were particularly fond of the butterfly design.
The Tales of the Heike - Burton Watson
The Haike were one of the four important clans that dominated Japanese politics during the Heian period (794-1185), along with the Fujiwara, the Tachibana, and the Minamoto. The Heike samurai clan defeated the Minamoto clan in 1161. Twenty four years later, betrayed and outnumbered three to one, were annihilated by the Minamoto at the great naval battle of Dan-no-ura in 1185. The surviving nobles bravely died fighting or committed suicide by jumping into the sea -graceful death considered preferable to a life in shame- however, according to one legend, their souls became butterflies. The Heike went down fighting, and so earned themselves the traditional Japanese admiration for brave and doomed warriors.
If you have an interest in Japanese culture and Bushido (The Way of the Warrior) in particular, The Tale of the Heike (Heike Monogatari) is the collective account of the 5-year Genpei War (1180-1185) for the control of Japan. One of the must-read epic masterpieces, a Japanese medieval classic, product of the compilation of oral tradition.
The most widely known version was first compiled by Yukinaga, a blinded warrior monk, in 1371. Although there have been several serious attempts of translations since 1918, a very complete one was made by the Stanford and Berkley scholar Helen Craig McCullough, published in 1988. Burton Watson (Translator) and Haruo Shirane (Editor) produced an abridged version published in 2006.