bobtail
 n.— «An American would be riding in the lead vehicle as commander and driving the last one as “bobtail”—a truck without a load that acts as a sort of sheepdog, rushing to help with any problems and making sure no one has drifted out of formation.» —“Truckers of Iraq’s Pony Express Are Risking It All for a Paycheck” by James Glanz in Safwan, Iraq New York Times Sept. 27, 2004. (source: Double-Tongued Dictionary)