The metaphorical use of bellwether to mean a human leader dates back to the fifteenth century.
Its modern use usually means a company that sets an industry standard.
A wailer is in fact a castrated ram or male sheep, and a bellwether is the leader of the flock.
He knows the routine, and he wears a bell so that the shepherd knows where his sheep are, because all the others will follow the bellwether anywhere without hesitation.
