Naruto Episode 185: A Legend Has Existed in Konoha
Episode 185 of 220 of Naruto, aired May 17, 2006, 25 minutes. Available with English subtitles and dub where released.
Episode summary
While Naruto trains in the forest, a mysterious creature latches onto his back. This legendary creature can't be removed until it becomes an adult, but that's only the beginning of Naruto's problems.