I've written about how the notion was popularized in the 19th century here. /u/Steelcan909 has written about how the original theory might have come up in the first place here. There's more to be said about the topic for sure but this might make for a nice start. Feel free to hit me up if you have questions.

It was popularised by the 19th century opera The Ring of Nibelung by Richard Wagner.

Before that, there probably were, indeed, horned helmets (though no solid archeological evidence can prove it), such as seen on the Oseberg Tapestry, from the mid-9th century in Norway. Despite that depiction, no actual helmet with horns ever were found that was dated from the viking age in Scandinavia. Those helmets most likely served ceremonial purposes, since wearing such an object in battle would be very unpratical