"Horn" is the generic name for both the hoofs and horns of animals. In Dwarf Fortress, hoofs and horns are functionally identical once stripped from their previous owners. Both are obtained by butchering
certain animals, although horns are often only present on the male members of a particular species.
They can either be carved into horn crafts, or used to decorate existing crafts at the Craftsdwarf's workshop. Doing so uses the bone carver labor.
Creatures with horns and hooves can be very formidable in combat, especially since most of them are several times larger than your average dwarf. They have the ability to cave skulls or bruise internal organs. It is best to approach all hoof/horn wielding creatures with caution.