The usual division is tidy. Deer grow antlers of bone, shed whole and regrown each year. Cattle, sheep and antelopes grow horns, a permanent bony core in a permanent keratin sheath.
The pronghorn breaks the rule. Its horn core is permanent bone rising from the skull, but the keratin sheath around it is shed after the rut, peeling off as a hollow, forward-branching case while a new sheath is already growing underneath from the skin covering the core. No other living mammal sheds a branched horn sheath in this way, which is one reason the animal sits in its own family rather than among the antelopes it superficially resembles.
The pronghorn is also the fastest runner in the Americas, sustaining speeds no living North American predator can match, a mismatch often explained by extinct Pleistocene cheetah-like cats. Female pronghorn usually have much shorter horns, and how regularly they shed their sheaths is less well described.