Bellwether

ˈbɛlˌwɛðər

noun

a person or thing that assumes the leadership or forefront, as of a profession or industry

The term 'bellwether' originally referred to a castrated ram (a wether) that wore a bell around its neck and led the flock of sheep. In a figurative sense, it now signifies a leader or indicator that others follow or imitate.

In cases, however, where it is believed that the leader and bellwether cannot be dispensed with, attempt after attempt is made nowadays to replace commanders by the summing together of clever gregarious men: all representative constitutions, for example, are of this origin.

Friedrich Nietzsche

Beyond Good and Evil

When the old bellwether at the head rattles his bell, the mountains do indeed skip like rams and the little hills like lambs.

Henry David Thoreau

Walden