The story unlocks once you make your call.
A myth. Horns came from an opera costume designer, not from Scandinavia.
Where it came from
Horned helmets did exist in Scandinavia: the bronze Veksø helmets from Denmark date to around 900 BC, nearly two thousand years before the Viking age, and were probably ceremonial.
The Viking with horns was born on the stage. For the first Bayreuth production of Wagner’s Ring cycle in 1876, costume designer Carl Emil Doepler gave the Germanic heroes horned and winged helmets. Romantic painters and, later, cartoons and sports mascots did the rest.
What’s actually true
The only reasonably complete helmet surviving from the Viking age, found at Gjermundbu in Norway, is a plain iron cap with a spectacle-like guard. No horns. In battle, horns would have been something for an opponent to grab.
Further reading
- Roberta Frank, “The Invention of the Viking Horned Helmet” (2000)
- Wikipedia: Gjermundbu helmet