天馬空を行く (tenbakū o iku: to advance unobstructed)
heaven + horse + unoccupied + to go
A Japanese informant tells me that 天馬 means a “very fast horse” or even “Pegasus,” the Greek winged horse. Although 空 literally means “unoccupied,” it metaphorically means “so fast it’s as though there’s no load.” So we should not conclude that the horse lacks a rider.