Bellwether

Bellwether \BEL-weth-uhr\ , noun;
1. A leader of a movement or activity; also, a leading indicator of future trends

How's you like to butt heads with this guy? (source)

This word dates to the mid-14th century, though as a surname it goes back to the 12th. It's a combination of bell + wether based on the practice of hanging a bell on the neck of the lead sheep in a flock (the wether). Today, a wether is a castrated male sheep, but in Old English weðer meant "ram," so the idea a wether being the lead sheep may make more sense.