bellwether

noun

1. a wether or other male sheep that leads the flock, usually bearing a bell.
2. a person or thing that assumes the leadership or forefront, as of a profession or industry: Paris is a bellwether of the fashion industry.
3. a person or thing that shows the existence or direction of a trend; index.
4. a person who leads a mob, mutiny, conspiracy, or the like; ringleader.

noun

1. a sheep that leads the herd, often bearing a bell
2. a leader, esp one followed unquestioningly

n.

mid-14c. (late 13c. in Anglo-Latin; late 12c. as a surname), from bell (n.) + wether; the lead sheep (on whose neck a bell was hung) of a domesticated flock. Figurative sense of “chief, leader” is from mid-14c.