History check!

Akuto- (13th Century Late Kamakura-early Muromachi period)

After the Mongol invasion of Japan, lower class samurai suffered serious economic problems. They became renegades without land & some became bandits. They defied both local administrators & the samurai government in Kamakura, challenging all forms of exsisting authority. These were the Akuto, bandit samurai- but not to be confused with the later Ronin. Akuto functioned as independent, organized econmoic group of warriors. They did not serve a lord, unlike the samurai who pledged loyalty to their master even in death.

Akuto are famous for using a giant sword called a Nodachi, the "horse cleaving sword". As the name sugests, it was a 6ft blade that can slay an oncomming horse & rider with one sweep! However, after such a strike the blade was severly distorted & later thrown away. If I make a guess, I think the fictional "Zanbatou/Zanpakutou" in anime derived from the Nodachi?