The Eight Immortals did not emerge as a group until the Yuan dynasty (1279-1368 CE). It is doubtful if they were indeed historical figures. Each of them has a personal mythology traceable to the Han (200 BCE- 220 CE) and Song (960-1279 CE) dynasties. They are also associated with Eight Trigram directions in the Classic of Change or Yijing 易经. As in here, Zhang is often depicted riding his magic mule backwards. His vehicle could be easily transformed into a piece of paper and folded to be kept away when he reached his destinations.
