Orochimaru
Orochimaru (大蛇丸), featured in the Japanese folktale Jiraiya Gōketsu Monogatari, is the archenemy of the ninja Jiraiya. He was once named Yashagorō (夜叉五郎) and was one of Jiraiya's followers but was overtaken by serpent magic. Having changed his name to Orochimaru, he gained the ability to turn himself into a giant serpent. He attacked Jiraiya and Tsunade and defeated them with his venom, only for another follower to save the couple's lives afterwards.
Orochi means "giant snake" or "serpent". The legend of Orochimaru has modern day correlations. Legend suggests the "power of the white snake" can be found in the fabled Ryuchi cave which grants the user with extraordinary powers, such as immortality, face stealing, and shape shifting. Japanese folklore speaks of a similar snake-like humanoid that stalks the fields every harvest time demanding a portion of the farmers crops.