Board Thread:Naruto Discussions/@comment-25113423-20150207211028/@comment-945885-20150216220931

I don't think the horns have anything to do with the bone abilities, as Hagoromo and Hamura also had them, and Obito and Madara both grew horns when they became Ten Tails' host. And the sacred rope belt is an Orochimaru deal; he, the Sound Five, and Sasuke all wear them.

I do wonder if the horns might be indicative of Senjutsu, however. Curse Seal users tended to grow horns, Juugo grows horns, and Kabuto grew horns.