The horns on the helmet is slightly different (more metal, then scaled horns), maybe a custom helmet (forged?)
http://scrawlfx.com/wp-content/uploads/2011/04/The-Elder-Scrolls-V-Skyrim_2011_04-18-11_005.jpg
http://bulk2.destructoid.com/ul/199017-/SpiderHero-noscale.jpg
It looks to me like it's probably a different helmet, note that the basic appearance of the helmet aside from the horns seems like it may be different as well, it's just that both have the horned setup, which we'll probably see a lot more of in Skyrim than we did in past games given thw Viking theme of the Nords. Even though Vikings with horned helmets may not be historically accurate, it doesn't change the fact that it's something people associate with them, and as a work of fantasy, Skyrim can play off that association without needing to worry about whether it's historically accurate.
And in the larger version of the image, it definitely looks like the thing in question is in fact part of the helmet, just a feature of this particular one. So why is it there? A designer thought it would look good, I'd guess.