Bellwether

['belweðə(r)] or ['bɛl,wɛðɚ]

(noun.) sheep that leads the herd often wearing a bell.

(noun.) someone who assumes leadership of a movement or activity.

Edited by Elise--From WordNet

(n.) A wether, or sheep, which leads the flock, with a bell on his neck.

(n.) Hence: A leader.

Typed by Clint