Brass horns were a rather seldom mutation of chaos beastmen. Their horn were hollow and made of brass. Unfortunately brass is not one of the hardest metals around and the horns were hollow. That did not inporve the damage they did (quite lessened it even), but the horns produced some kind of music when charging. There was a whole unit of beastmen in service of Slaneesh, all of them sporting brass horns. They were called the Benny Goodman Big Band, due to their general.