The Vikings may not have worn horned helmets, but the ancient Greeks had helmets covered in boar tusks and the Dayak of Borneo had helmets covered in fish or pangolin scales.
The stereotypical Viking helmet has large horns sticking out either side. There is no historical evidence or physical artefact to support that design. But that’s not entirely surprising, given that very few Viking helmets have survived to the modern day: only half a dozen in total, and many of those just fragments.
The most complete Viking helmet, reconstructed from nine different pieces, is the Gjermundbu helmet:
You may notice a distinct lack of horns, or even a place to attach horns. That’s not to say that horned helmets didn’t exist at all. There are examples from outside the Viking age, like this helmet discovered in Denmark and dating back to the Bronze Age:
But the popular image of the horned Viking helmet probably dates back to the Viking revival. In the 19th century CE, as Romanticism swept across Europe, the Viking Age became a valorised and romanticised source for creative endeavour. Der Ring des Nibelungen, the epic opera cycle of Richard Wagner (he of Tristan chord fame), drew on Norse mythology… and its first full production featured horned helmets. Inasmuch as there’s a single cultural origin point for the Viking horned helmet idea, that’s probably it.
Using animal parts for protective head gear isn’t entirely in the realms of fiction and ceremony, though. In Homer’s Iliad is the following description:
Meriones found a bow and quiver for Ulysses, and on his head he set a leathern helmet that was lined with a strong plaiting of leathern thongs, while on the outside it was thickly studded with boar’s teeth, well and skilfully set into it.Iliad, trans. Samuel Butler
More than one of these Mycenaean boar’s tusk helmets still exist. The one pictured at the top of the page dates back three and a half millennia and would have used the tusks of forty or fifty boars.
My personal favourite animal helmets have to be the katapu. These helmets were made by the Dayaks in Borneo. The base of the helmet was rattan cane, and for decoration / protection they were sometimes covered in fish scales or even pangolin (spiny anteater) scales. I think you’ll agree that scales like this would make a truly excellent helmet: