Bellwether

A wether is a castrated male sheep or goat. A bellwether was originally a wether who led a flock of other sheep; a bell was placed around his neck to help locate the animals.

Source: tatoeba (8658032)

Although the male goat kids are often culled because they will never produce milk, they are sometimes castrated instead and allowed to remain in the herd as a "wether". With a bell around its neck, such a "bellwether" is usually the leader for the rest of the herd.

Source: tatoeba (10614282)

Several old ladies forthwith proclaimed their intention of following him; but, as one or two of them were deaf, and another had been threatened with an attack of that mild, but obstinate complaint, dementia senilis, many thought it was not so much the force of his arguments as a kind of tendency to jump as the bellwether jumps, well known in flocks not included in the Christian fold.

Source: wiktionary

Mortgage delinquencies often act as a bellwether for a forthcoming recession.

Source: wiktionary