The oldest representations of Lucifer in art date back to the Byzantine era.
In the church of Saint Apollinarius in Ravenna is a Byzantine mosaic from 520 AD which depicts the parable of the sheep and goats from Matthew's Gospel.
Jesus is pictured without a beard, a tradition common in early Christianity.
To His right is an angel with red wings and a red robe. This is likely Archangel Michael. Below Him are the sheep, which, according to the parable, will enter the Kingdom of Heaven.
To Jesus' left is an angel dressed in blue. Beneath him are the goats, representing those who will go to Hell.
According to the historian Jeffrey Burton Russell in his book Lucifer: The devil in the Middle Ages, this blue angel is actually Lucifer. The image of Satan as a humanoid creature did not begin until much later in the Middle Ages, and most likely influenced by ancient images of the god Pan.