How did the depiction of Vikings as wearing horned helmets become so ingrained in Western culture?

So... back in the 18th and 19th centuries there was a sort of "Norse history revival" in much of the world, prompted in part by the earlier research (translations of the Gesta Danorum, the Edda and archeological finds to name a few) that had caught the interest of academics of the day. This happened right smack in the middle of the romanticism which already had a tendency to glorify the past and make heroes of old dead people. To make matters even worse, nationalism was on the rise everywhere. In this climate a young composer grows up in Germany. He goes on to write, among other things, Der Ring des Nibelungen which features valkyries and shit. Since it was an opera he needed a fat lady on the stage and actors and stuff, they needed costumes for the newly founded Bayreuth music festival so he calls an artist and designer called Carl Emil Doepler.

Carl Emil Doepler designs the costumes and puts horns on the helmets. Where he got the idea for this we don't know. To my knowledge no one asked him where he got the idea. I know he drew hundreds of sketches and drawings for these costumes but that's it. Anyway. It could have ended there, but it didn't. A few years later, in 1881 specifically, opera director Angelo Neumann bought the entire original set of Der Ring des Nibelungen and set up one of the most famous versions of it. It was hugely successful and played in Paris, London, Austria, Rome, fucking everywhere. They did over 50 cities in two years. It was like a god damn rock band trying to push their latest album, but 1880's style. People fucking loved it and the piece became iconic and set the trend for how we were to think of vikings for the next 150 years.

TL;DR - The fucking Germans did it!