"Horns" (sensu lato) are what bovids have on their heads; "horn" (sensu stricto) is the outer covering or sheath of keratinized skin on bovid horns. Keratin horn does not preserve as a fossil, except perhaps in permafrost.
In bovids, horn cores are living bones (that is, they are vascularized and they grow). Horn (keratin) is not vascularized, though new keratin is laid down to keep up with bone growth. Bovid horns are not shed seasonally as with cervid antlers, nor is the keratin sheath shed annually as with the antilocaprid pronghorn antelope.
Bone horn cores have internal channels and buttresses, and have no distinct margin between core and bone of the skull. The keratinized skin covering of the horn core is without interior channels or buttresses. The keratin wall tapers to a feather edge and usually shows growth rings on the exterior.