Muramasa was a nasty dog appearing only in the Shin Gaiden story.
It's unknown where he came from but he took over Reiji Hidaka's pack by using Shin's Mother as hostage while he was away fighting Akakabuto. He ruled in cruel and harsh way, giving his minions only little food. His minions apparently killed Reiji Hidaka's two sons Tommy and Sabu when they tried to leave to get help. Muramasa treats Shin Hidaka, the real but young leader, as his slave. Shin is very afraid of Muramasa and obeys him. Muramasa has Shin's mother as his prisoner and threatens her life if someone from the pack tries to disobey him. Gin comes with Ōu dogs to the area and after hearing what happened, they help the local dogs to get rid of Muramasa. Muramasa orders Shin to kill Gin but Shin eventually finds new courage and attack Muramasa instead. In the end, it is Akame who defeats Muramasa by cutting deep wound on his neck.
- Some fans believe Muramasa to be a Pitbull mix