It's some kinda Hashirama arm. Evil yesterday posted some teasers about it. Someone guessed Zetsu arm but Evil said something like "No, think about the source" and posted a picture of Hashirama in a later post.I also believe that Naruto might be using a puppet arm. If it was a perfectly healed version of his own arm like he did for Kakashi and his eye why bind it up in bandages??
Perhaps the Hashirama clone in Madara's hideout?