The Shahi Gandhara culture in south Afghanistan-Pakistan became a part of one of the Han, (Khan) Khanates. The ruler, like other rulers, needed coins; after all, most kings and parliaments do not want to collect tribute (taxes) in the form of eggs or cheese. The local Khan wanted a proper image for his coinage. Since every Mongol (Han) conquest had been won from the back of a horse, a horse and a ruler riding it became the natural choice for a symbol of the new ruler in Gandhara.
A coin with a kneeling horse on one side and a rider with a staff mounted on the back of the horse on the opposite side is how the coinage was cast in Gandhara. Quoting from Wikipedia: