bellwether [ˈbɛlˌwɛðər] n.
1.) The leading sheep of a flock, on whose neck a bell is hung.
2.) fig. A chief or leader. (Mostly contemptuous.)
3.) fig. A clamorous person, one ready to give mouth. (Sometimes opprobrious.) (O.E.D. 2nd Ed.)
Etymology: Middle English bellewether, from belle, bell + wether, a castrated ram.
"'Tell me dear child
who is that officer? The son of Atreus
stands a head taller, but this man appears
to have a deeper chest and broader shoulders.
His gear lies on the ground, but still he goes
like a bellwether up and down the ranks.
A ram I'd call him, burly, thick with fleece,
keeping a flock of silvery sheep in line.'
And Helen shaped by heaven answered him:
'That is Laertes' son, the great tactician,
Odysseus'" (The Iliad by Homer, Robert Fitzgerald (trans.), 1974).