Horned helmets are usually associated with the Bronze Age, especially on Germanic helmets. There have been some finds of horned helmets in Sweden but these pre-date the Viking Age. The Japanese and some other Asians also had them. The association with Vikings came about through literature and plays in the 19th century. There is no evidence that Viking warriors ever used horned helmets in battle (they'd be a liability, actually, especially if metal [the weight] but even animal horn would "catch" a sword or axe).