In military terms a Shavetail usually refers to a Second Lieutenant who is not very experienced in Army matters. The saying comes from the practice in the army of shaving the tails of newly broken pack mules to distinguish them from seasoned ones. Good to know when you were picking an animal from the group.
Bell Sharps is another grading system,
and another whole fashion statement!
As the mules were trained and passed muster for performing various duties, their achievements were noted by again altering their tails. When needed, the soldier knew which animal to chose for which specific task.
One bell was a pack mule, two bells was pack and ride, three meant he'd pack, ride, and drive.
Looks like the one on the left is training the one on the right
as to the proper etiquette of driving.
Bell Sharps is not to be confused with Bellwether,
literally a whole different animal.
A bellwether is one that leads or indicates trends. The term is derived from the Middle English bellewether and refers to the practice of placing a bell around the neck of a castrated buck or ram (a wether) leading his flock of goats or sheep.
My Bellwether, Cowboy. Best Nubian goat in Weld county.