Vikings never wore horned helmets. Blame Wagner.
Not a single Viking helmet with horns has ever been found; warriors of the years 800 to 1050 wore plain iron or leather caps. The famous horned helmets from Veksø in Denmark are about 2,000 years older and were Bronze Age ritual objects. The cliché arose in the 19th century, partly through costumes for Wagner's 'Ring' in 1876.
Useful: When you picture an era, ask which century the picture actually comes from.
Source: en.wikipedia.org