@Seek pretty much nailed this one to death, and very nicely. The horned helmet stereotype started with the Romans, who attributed such helmets indiscriminately to all kinds of Northern barbarians; later this was reinforced by some archaeologists digging up a Viking helmet near a couple of drinking horns and assuming that they had once been one piece.
Such helmets were not only impractical, they would actually be dangerous if worn in combat (a helmet with such a feature could be grabbed by an enemy who could pull the wearer off-balance. Worse yet, an overhand blow to the head could snag on the joint between the helmet and the horn, greatly increasing the chance that the blow would bite deep.) Horned helmets may be replaced by the (equally unhistorical) winged helmets, especially when the work wants to present the Vikings as noble rather than barbaric.
The myth of the horned helmet was, as Seek duly notes, later revived by Victorian/Wilhelmine-era composer/theater director Richard Wagner in his staging of his extremely popular opera/dramas concerning Nordic mythology such as the Der Ring des Nibelungen which is loosely based on the Norse sagas.
From there, the horned helmet became a common element in the hyper-romantic literature popular throughout the Victorian period and from there to silent film, then to sound and color film right up to the 1960’s when reality set in and it became a joke. But it is still seen today, nevertheless, because there is nothing cooler than a Berserker wrapped in arctic wolf-pelts topped with a bovine-horned helmet.