origin:  Old English bellen = "roar" + weðer= "ram"
We think of sheep as being followers, but if you look closely you may spot one who is leading the pack, charmingly marked with a bell around his neck (allowing the shepherd to track the entire flock).
From this we extend the meaning to mean "leader of the pack" or an individual who sets the tone for things to come; a leader followed without question: frontrunner, trailblazer, visionary.
The dark side of this trait would be a ringleader of crime, conspiracy, or mutiny.
Can you name a bellwether? What qualifies them as one?