Also have a look at Asia, Africa and The America`s. I believe it is something you see in a lot of cultures, a part of psychological warfare to scare the enemy before and in battle. So I believe when you will look more into it, I`m very positive it is more a common trend and of all times. Romans, Vikings, Samurai, Azteks...and many other used them and it works...still does, think of horror movies where they apply it a lot.
Thanks Lord Z. I am aware that other cultures used them, but not to what extent. I decided to focus mainly on Rome, Parthia, Gaul, Greece, etc, because my research is directly related to these peoples. :D