Mustaches appear in the Bayeux Tapestry commemorating the Norman conquest of England in 1066. Earlier visual references to mustaches appear in Roman, classical era statues that were likely copies of earlier Greek art (but whether the mustaches occurred in the earlier pieces cannot be known with certainty). See the Dying Gaul and the Ludovisi Gaul Killing Himself and His Wife. It appears that mustaches were popular with warriors of various Celtic-speaking groups, but it may also be a Roman-era generalization (even so, it is likely based on some part of reality).

Perhaps another redditor will have earlier examples of this type of facial hair.