OK I'm stubborn so I had to look this up 
It is true.... vikings did not wear horns in their helmets.
(...)
Anyway.... it is a stereotype and I like them better with horns
Yep, I learned vikings had no horns when Age Of Empires 2 was in development
Their vikings HAD horns, a marketing decision, not an historical one. It surprised me to know they didn't in fact.
But I also prefer them with horns, even if it's not accurate
