What eight-headed serpent did Susanoo defeat?

Susanoo defeated Yamata no Orochi, the legendary eight-headed and eight-tailed serpent of Japanese mythology.

The story appears in both the Kojiki and the Nihon Shoki, though the versions differ in some details. After being expelled from the heavenly realm, Susanoo descends to Izumo and encounters an elderly couple mourning their daughter Kushinadahime. They explain that the serpent has devoured one daughter each year and is now coming for the last one.

Susanoo agrees to rescue Kushinadahime in exchange for marrying her. He turns her into a comb for protection and has the couple prepare eight vats of strongly brewed sake. When Yamata no Orochi drinks from all eight, it becomes intoxicated. Susanoo then cuts the serpent apart and discovers a sword inside its tail.

That sword is identified as Kusanagi-no-Tsurugi, one of Japan’s Three Imperial Regalia. Kitsune, kappa, and tsuchinoko are also famous Japanese legendary creatures, but none is the serpent slain by Susanoo. “Yamata no Orochi” is often translated as “eight-forked serpent.”