Of (overly pedantic) note: It's actually "bellwether". A wether is a castrated ram. (Like a steer, speaking bovinely.) In a flock, a wether would be outfitted

Of (overly pedantic) note: It's actually "bellwether". A wether is a castrated ram. (Like a steer, speaking bovinely.) In a flock, a wether would be outfitted with a bell to serve as the lead sheep and herald the flock's motions, which is where we get the modern-day term.