The most historically plausible explanation that I have found is that the idea of Vikings wearing horned helmets originated from the mingling of armour worn by the 1st century Northern-European peoples.

In his book Germania (circa 98 CE), the roman author Tacitus described that some Gauls wore helmets of metal and leather headgear. The book was eventually lost during the Middle Ages but luckily one manuscript was found in the 15th century. In its first print, an excerpt describes the Gauls has wearing helmets of great height, decorated with horns and images of animals. I am unacquainted with the reason why there is a discrepancy here with the original text. Could the gear’s literal description have been lost in translation?

Anyhow, aware that Gauls are not a Germanic but a Celtic people, the German humanist Heinrich Bebel established in 1500 that early Roman writers also meant Germans when writing about Gauls or Celts.

In his Germania Antiqua (1616), the German historian Philipp Clüver, illustrated, based on the authority of Tacitus and other Roman writers, the early Germanic peoples wearing horned helmets.

In early 1875, Richard Wagner and his costume designer, Carl Emil Doepler, attended a play by Henreich von Kleist where early Germans were depicted wearing horned helmets. The troupe acting the play was known for having historically accurate costumes, which, in this case, were most probably based on Tacitus’ descriptions and Clüver’s illustrations.

Carl Emil Doepler proceeded to introduce, based on Kleist’s play and his own reading of Germania, a cow-horned helmet to Wagner’s Der Ring des Nibelungen, which was first played in 1876.

This is when the armour worn by the 1st century Germanic peoples was blended with Norse mythology and Viking popular history.

At a time when there was growing nationalism in Europe and an empire-building sentiment, the horned helmet well symbolized war, battle, conquers and the like. England and Germany popularized the idea, trying to link Vikings back to their own cultural heritage. More and more art and literature started portraying Vikings with horned helmets.

And that same time, the concept of a Viking age saw daylight. A Danish and Swedish historian published articles concerning the Scandinavian age of naval raids and pillaging and the idea of an age dedicated to Vikings went hand in hand with the idea of horned helmets.

The Viking horned helmet then found its way through western popular culture from that time all the way to Loki’s headpiece in The Avengers movie.