The Orochi

I wasn't sure if this was supposed to be the Orochi at first, but there aren't many other prominent 8-headed serpents in Japanese mythology, and the Hydra had 9 heads, so I'll go with the former.

The Orochi, or the Yamata No Orochi (Eight-Branched Great Snake), was an eight-headed and eight-tailed serpent sent down to Earth to destroy it. It was kept at bay by the Foot-Stroking and Hand-Stroking Elders, by sacrificing their eight daughters (one for each head) to it. When they were due to send their last daughter, Wondrous Princess Inada, to it, the disgraced deity Susano-Oh- exiled from heaven for tricking the sun-goddess Amaterasu- came to their aid.

He told them to make a fence with eight gates, and put platforms behind the gates, holding up eight casks of refined liquor. When the monster came, it drank from each cask- one head to each one- and got so drunk that it fell asleep. To finish it off, Susano-Oh came by and decapitated each head and each tail, but his sword broke when he went to slice the middle tail. Confused, he used his broken blade to tear it open and found a great blade within it- the Kusanagi-No-Tsurugi, or Grass-Mower Sword- and took it for himself.

King of Fighters fans may recognise this story as being the mythological set-up for KOF's '94-'97, where its main protagonist Kyo, his rival Iori, their mysterious benefactor Chizuru and the big boss of KOF '97 Orochi were human avatars of both the giant serpent and of the Three Sacred Treasures of Japan- the Kusanagi Sword, the Yasakani Jewel and the Yata Mirror.

The treasures themselves exist, and are said to be found at Atsuta Shrine in Nagoya (the sword), the Imperial Palace in Tokyo (the jewel) and at the Grand Shrine of Ise in Mie Prefecture (the mirror), though the authenticity of at least one treasure is in doubt. Still, it would be a neat sight to see them, should you get the chance to do so (especially the jewel- they don't usually let the public into the Imperial Palace itself)