A Legend from the Hidden Leaf: The Onbaa!

is episode 185 of the original Naruto anime.

Synopsis
A legendary creature, the Onbaa, attaches to Naruto's back and causing havoc wherever Naruto goes. Despite Naruto's efforts, it remains firmly attached to Naruto's back. Tsunade mentions that Jiraiya once went on an expedition when they were kids to find the Onbaa. Tsunade refused to believe that he could find one, however, Orochimaru and stated that he was "betting on Jiraiya this time" before quickly turning around so that no one could see him and began blushing with embarrassment at his sudden outburst and for the fact that he never normally agreed with Jiraiya. Jiraiya then headed into the woods around Konoha and returned sometime later having written a report on Onbaa's, claiming that he had fought one, although Tsunade paid him no heed at the time, believing him to be a fool. The Onbaa continues to increase in size, eventually becoming larger than Naruto himself. However, during a mission, it aids Naruto, causing Naruto to reconsider their relationship, but the Onbaa's mother appears, driving Naruto off and claiming her offspring. The child manages to calm the mother, and the two leave. As Naruto returns to the village, most of the inhabitants have Onbaa on their backs, something Naruto finds absolutely hilarious and advises Tsunade to fight the Onbaa like Jiraiya said.