It never occurred to me to ask the question: 
Vikings did not wear horned helmets. According to [Cecil Adams], “contemporary Viking era artwork shows roughly half of Vikings in battle bareheaded, while the rest wear unremarkable dome-shaped or conical helmets.” The idea that Nordic invaders of the ninth and 10th centuries wore headgear festooned with ox horns developed a thousand years after the fact, when a Swedish artist illustrated them as such for a poem based on an old, Icelandic saga.
And, evidently, there’s a good reason why:
No self-respecting Viking warrior ever wore a horned helmet in battle–they weren’t that dumb. As anyone who has done any slaughtering can tell you, horns provide nothing more than a good handhold to steady your work while you’re slitting someone’s throat.
Keep that in mind next time you enter hand-to-hand combat.