A few people have laid claim to being the originator of the devil horns. Ronny James Dio claimed he stole the horns from his grandmother. Gene Simmons (complete douche, and usurper of all things good) says he invented it on the cover of their album "Love Gun." Sorry but, John Lennon clearly is the first metalhead. Just look at him up there double fisting some horns. And if you think it's just a coincidence here's more proof.  

 If that's not proof enough they even animated it.

Sorry Dio (rest in peace) you didn't invent it. Gene Simmons you're a douche and don't deserve credit for anything you've ever done. Just ask the devil.