Should be the sign for plasma canons. Nearly every CSM have horns on the helmet. Terminators so big, they must have trouble looking around. (but it looks soooo cool ) But are they growing naturally ? (warpilly) or do they hunt funny horny creatures to decorate themselves?
You would be surprised... Spoiler Mo'rcck: https://en.wikipedia.org/wiki/Michael_Moorcock Phraz-Etar: https://en.wikipedia.org/wiki/Frank_Frazetta An'sl: https://en.wikipedia.org/wiki/Bryan_Ansell More seriously, though, it's just because of the visual association with the pernicious powers and their minions.
This is why i said it depends on the marine/person there are horns/tusks that are glued on the armor (Ahrimans horns) and there are horns/tusks that seem to be the result or the warp or mutations(CS?!)
Backpack: I always felt the CSM had a steampunk thing going on, it's all about style! Horns: They're evil...horns=evil. Nuff said.
