"The Enemy of the Byakugan" (白眼の天敵, Byakugan no Tenteki) is episode 154 of the original Naruto anime.
With Neji unable to see through the mist, the group loses the advantage of his eyesight. He is tricked into thinking that random objects are real people by a mysterious jutsu. Rock Lee manages to hold Raiga off by fighting while still unconscious. When a mysterious case is knocked off of Raiga's back, Raiga becomes weaker and is eventually defeated by Naruto. When the gang opens the case, they discover a young boy named Ranmaru, who has been riding on Raiga's back the entire time. Ranmaru reveals that Raiga cared for him, and that he helps Raiga fight by serving as his "eyes".