Check out this mag-ni-fi-cent mural of the northwestern deity Tyr, God of war, painted in Germany in 1850. Besides the fact that he is missing an arm and has the physique that epitomized Aryan masculine ideals of the 19th century, the other obvious feature is his horned helmet. It was from this milieu, my friends, that the Viking horned battle helmet idea arose: because the actual Vikings didn’t wear them.
The Viking era (roughly 800-1100 CE) was romanticized in the European age of Nationalism. Germans connected their own history to an imaginary Scandinavian past, when the bellicosity of the Vikings who raided other lands was celebrated as an expression of powerful manhood. White nationalists today do the same. There are all kinds of problems with this, but here we are focusing on the sartorial misunderstandings: the horned Viking helmets.
In 1825, Esaias Tegnér finished his epic poem which echoed the Icelandic sagas of yore, and it became super popular throughout Western Europe. The _Frithiofs saga_ was printed in different languages, and the illustrator, Gustav Malmström, showed the Vikings with horned helmets. The idea spread from there, even though Malmström probably didn’t use Scandinavian sources (he might have borrowed from late Medieval fantastic headgear sometimes worn at tournaments — but this was not a Viking thing).
There were horned helmets from Scandinavia, however (found after Malmström died) — but these were too delicate to have been used in battle, and more importantly, date a thousand years earlier than the Viking period. You can see an example of one of the “Viksø” helmets in the second slide, which dates to the Nordic Bronze Age (ca. 1100 BCE-900 BCE).
Precious little armor survives from the actual Vikings. Many archaeologists think the head protection of most Vikings would have been made of leather. However, the exceptionally rare metal Viking helmet remains (like the Gjermundbu helmet on the third slide from the 10th-century) are decidedly lacking in the horns. Good thing for the warriors, too, because wearing crazy add-ons like that to helmets would definitely have been a disadvantage in combat.
Sources: Tyr, Gustav Richter, Tyr der gott der Schlacht und die Farht Nach Walhalla, 1850 Neues Museum, Berlin, Deutsche Digitale Bibliothek, Photo: Uwe Gaasch 2000, public domain, See chapter nine of The Vikings: Facts and Fictions, by Kirsten Wolf and Tristan Mueller-Vollmer, ABC-CC L