Beside the Prankh helmet in Vienna the only original horned crest known to me is the sample from the Churburg, Southern Tyrol, Italy. The Churburg holds a large collection of armour including the oldest piece of mediveal plate armour. And it has this crest. The pedigree is unknown to me, as a complete publication is not available today.
(H.T.photo)
As you see, it's made like a cap which simply was put over the (sugarloafed?) helmet. Please compare that with the effigy of Burkhard von Steinberg in the previous post by Mike.
The european antique has around 10 pieces (more or less) of fixed "horn" crested helmets, from the mediterrean plus two examples from Denmark.
The danish samples are bronze age and where used for some sort of cereminial plays (which is depicted with miniatures!)
http://en.wikipedia.org/wiki/Veksø
And there are several samples from Greece and from italy with a broad range of dating. All these "horns" are flat.
Northern Italy, around 400 BC
http://photodiarist.com/blogpics/2007 04 06/16.jpg
Greece, around 600 BC, I think.
At last one picture from our local Germanisches Nationalmuseum, a northern italic piece of around 500 BC. These protuding devices maybe the sockets for horns (from horn or from metal).
H.