During the days of mule trains the Army actually had soldiers trained as muleskinners that were responsible for packing and transporting supplies when the unit was on the march. When the muleskinners got new, untrained mules into the unit they very seldom had time to train them properly to be effective members of the mule team. So the new mules were interspersed with the trained mules during the march with the muleskinners watching their new stubborn charges closely. Because of one untrained, stubborn, do your own thing mule could ruin the entire train the muleskinners would shave off the hair around the tail area so the new unruly mules could be spotted from behind and possible trouble could be averted.
Because of their resemblance to the new, untrained. stubborn mules who would do their own thing, brand new 2nd Lieutenants were referred to as "shavetails".
BEAR (BDY)