The earliest known art of Satan that I have encountered is the mosaic in the Basilica of Sant’Apollinaire Nuevo in Italy. In it Lucifer is depicted as a “blue angel,” a standard angel except colored blue.

In this piece of art, Satan is pictured next to Jesus on judgement day behind the goats (those who are not saved). He is shown as a radiant angel, with a standard Halo and everything except that his robes are colored blue well the robes of another angel in the painting are colored red. This fits in line with the Augustinian view of Satan which held that he maintained his angelic qualities yet that he was fallen.