Possible flag or banner of the Ottoman Empire (or Ottoman army) according to Hieronymus (aka Jérôme) Bosch (crwflags.com), the painter lived in the Middle Ages (c. 1450–1516) in Brabant and some of his paintings show Ottoman flags. These are generally red with a white crescent. It was the end of the XVth century, when the Ottomans had just conquered Constantinople and thus ended the Byzantine Empire, that was a shock for the Christian world.
Own work (crwflags.com)
BlinxTheKitty