A bellwether was a castrated ram who led the sheep to slaughter
One animal in a flock of sheep usually established itself as leader, and in the absence of rams, it would normally be a wether. It was useful for the shepherds to know where it was, and which one was the leader, so it was given a bell. I don't know, but imagine that the other sheep learnt to follow the bell as well as the wether. But it was not just for leading to slaughter, though it might certainly be used for that.
Today in Switzerland, it seems that many of a flock of goats on the mountain will have bells, and several cows. I have never asked them what the command hierachy is.

Rod