I was reading some the De re militari primary source documents and I found a very unusual passage. In a fight between Henry V and the city of Cologne

"There was in the emperor's army a corps whose armor was made of horn and so could not be pierced by iron. When these removed their armor, however, in order to get a little air, for it was very hot, they were immediately covered with arrows, and all but six fell on the spot."

The rest of the document can be found here. http://www.deremilitari.org/resources/sources/cologne.htm

Any idea what this horn armor may have been? I have never heard of such a thing, but I am not an expert. :P

Sorry, I should have been more clear. It is not Henry V of England, it is the Holy Roman Emperor by that name. The year the event took place was 1114, though it was not written in the chronicle for some half century IIRC.

I would guess this could be some sort of scale armour made of horn plates. One Anglo-Saxon helmet - the Benty Grange - was supposed to have had horn plates instead of the usual iron spangenhelm plates.