Vikings wearing helmets with horns has been stereotyped by cartoon shows and animated depictions of vikings. Vikings are also portrayed as beasts and savages who looted and raided innocent shrines and institutions. But archeological evidences prove that vikings wore plain helmets and traded a fair bit as well. So where did this pre-conceived notion come from?
It started with the vikings themselves...
Helmets with horns?
Depictions of an Iron Age date exist featuring people with horned helmets/heads, such as upon the Golden Horns. Similar images are also known from the Viking period itself.
In the Oseberg burial from Norway, which dates to the early Viking period, a tapestry was found on which horned helmets are also depicted. Does this prove that all Vikings wore the famous helmets with horns? The answer is probably not. However, there is some evidence to suggest that certain warriors wore such headgear. The horned figures on the Golden Horns are berserkers. These were wild warriors, who threw themselves into battle in a trance-like fury. We are also familiar with them from the Icelandic sagas, in which they are amongst the most feared of all Vikings.
It is also possible that such headgear was worn for display or for cultic purposes. In a battle situation, horns on a helmet would get in the way. Such helmets would also have caused problems on board the warships, where space was already at a premium. In addition, none of the contemporary sources mention Vikings wearing horned headgear.
The idea that all vikings wore these horned helmets is a more recent invention...
The popular image of the strapping Viking in a horned helmet dates back to the 1800s, when Scandinavian artists like Sweden’s Gustav Malmström included the headgear in their portrayals of the raiders. When Wagner staged his “Der Ring des Nibelungen” opera cycle in the 1870s, costume designer Carl Emil Doepler created horned helmets for the Viking characters, and an enduring stereotype was born.
You can blame the early ages of archaeology when differences between the different northern cultures and time periods were not well understood.
Horned helmets did exist in the bronze age, the Vikso helmets from Denmark an obvious example though these weren't known when the early depictions of Vikings with horned helmets were made. Mycenaean helmets judging from art from the period also had horns, possibly actual animal horns if the helmets made of boar tusks are any indicator.
Such helmets likely derive from early societies where horned animals were venerated. A series of deer skulls with antlers, modified to be used as hats were discovered at Star Carr in Yorkshire dating back to the stone age. Some had holes drilled into them in front of the antlers, presumably as eye holes.
Horned helmets of heroes or gods appear in Anglo-Saxon art such as on some of the panels on the Sutton Hoo helmet. They also appear in later Norse art.
Horned helmets appear in the Iron Age where Samnite and Celtic warriors wore them, probably an archaism from the earlier Bronze Age. They also appear in Celtic art and Roman depictions of Celtic warriors as well as Etruscan art.
There are also Bronze Age helmets from Greece with bronze wings of the Chalcidian type.
So helmets with horns or similar decoration are well attested.
My guess is that the classic Viking horned helmet is based on finds like those from Bronze Age Crete - the Horned God and Ingot God. Both have headresses/helmets with bulls horns. The examples from Viking art tend to have balls on the ends of the horns whereas these Cretan statues have sharp horns which is what we see in the classic Viking depiction.
I suspect the horned helmets depicted on the Golden Horns of Gallehus are actually horned entities similar to the Celtic Cernunnos. One is holding a circle which is similar to the torc Cernunnos holds and both are next to a collection of animals which Cernunnos is also associated with. There are two warriors nearby holding shields showing that the circle held by the horned figure is not a shield but an empty circle.