An onbaa is a cat/sloth/bear/monkey-like animal that lives around Konohagakure. Jiraiya encountered one when he was a child and later wrote a story about it, a tale so gruesome that Shizune was frightened. Tsunade had always assumed he was lying. When a baby onbaa ("Onbu") adheres itself to Naruto Uzumaki's back, she realises that the story was true.
Onbaa are quite strong, as nobody in Konoha is able to release Onbu's grip on Naruto. During their time together, Naruto discovers that Onbu has a voracious appetite that can apparently not be quenched. Onbu matures rapidly, growing to a size several times larger than Naruto during their time together. When they are found by Onbu's mother, she attacks Naruto to try to protect her child. Naruto is almost killed but Onbu, finally dislodged from Naruto's back, protects him. They say their goodbyes and Onbu disappears into the forest with its mother.
Soon after Onbu leaves the village, a flock of hawks drops several hundred infant onbaa onto the villagers of Konoha. Tsunade begs Naruto to tell her how he removed Onbu from his back. Naruto joked that she should have a sumo match with it like Jiraiya did, angering her.