This incident happened to Robin O'Brien of Georgia. He was working for the Yellowstone Park Service, packing out on the though fare trail. A large bull elk come around the bend on the same trail, saw the horse and mules and apparently thought they were other elk. He lowered his head, snorted and pawed the earth ready for a fight. Robins animals spooked. Two of the mules, tied head to pack saddle, ran on either side of a tree - breaking the packs - scattering food and supplies - and crashed unto each other, breaking free from Robin and each other. All 3 mules lit out back the way they had come. Quite the rodeo. The elk disappeared fortunately - but Robin spent quite a while gathering up food, supplies and th long gone mules.