Naruto, sasuke, Sakura, neji, Lee, tenten, shino, hinata, Kiba, shikamaru, ino and choji after the forth Shinobi war are reborn as pokemon and they voluntarily join ash's team at Arceus's request to help the chosen one and aura guardian ash Ketchum
And regarding the Shinobi the author can decide what kind of pokemon the Shinobi are reborn as.