He was a ninja from the Fūma clan who fought Jiraiya in the past, his corpse found by Nagato years later and converted into the first Animal Path. In the anime, the ninja is possibly the basis of the antagonist from Jiraiya's Tales of a Gutsy Shinobi, with his fight with Jiraiya almost completely identical with the fight depicted in the novel.
The ninja's hair was a light green colour when he was alive, tied back in a long ponytail with a fringe hanging down from the right side of his head. He received a scar on his forehead during an encounter with Jiraiya, which was usually obstructed by the Animal Path's forehead protector.
This body belonged to a member of the Fūma clan, due to this many citizens of Amegakure believed Pain to be from the Fūma clan.
Like other corpses, the Animal Path's hair was dyed orange. It had a large bridge piercing through its upper nose which was attached on each cheek with a circular stud, one stud on its upper nose, six studs around the bottom of its face, and one stud on each ear. It does not have eyebrows. While being examined by Konoha, the body was shown to have multiple piercings on its chest and legs as well. As a mark of Nagato's control, the Rinnegan appeared in its eyes. [Source: wikia]
You can download this Naruto paper model here: Naruto - First Animal Path Free Papercraft Download