Satan has been portrayed in art for way more than 500 years. From ancient times people wanted to portray evil in some way and they made drawings of something similar to Satan. They mostly portrayed deformed animal-human like creatures with often pointy horns and pointy sharp nails.
Example of Satan's "image" by codex gigas (Satan's Bible):
Satan and God are the only beings that are mentioned and drawn throughout history. While Satan is believed to have interacted more with humans than God, since even popes of those times secretly believed in him and in some cases it is believed Satan told popes what to do.