Check out this post from Man Alive.

When the word demon comes to mind we automatically think of red, scaly creatures with horns, wings, and pitchforks. Google “demons” and you will get just that. Funny that nowhere in the Bible does it describe demons physically. So how did we get this perverse idea of what demons look like and why do we continue to view them as such? part of it is probably the demons getting you to think that’s what they looked like if they exist at all. Think about it, a demon’s goal is to deceive you and make what is evil look good. If demons really did look like that and would appear to people, people would be afraid and most likely turn the other way.

The Following is taken from the preface of THE SCREWTAPE LETTERS by C.S. Lewis

I HAVE no intention of explaining how the correspondence which I now offer to the public fell into my hands.

“There are two equal and opposite errors into which our race can fall about the devils. One is to disbelieve in their existence. The other is to believe, and to feel an excessive and unhealthy interest in them. They themselves are equally pleased by both errors and hail a materialist or a magician with the same delight. The sort of script which is used in this book can be very easily obtained by anyone who has once learned the knack; but disposed or excitable people who might make a bad use of it shall not learn it from me.

Readers are advised to remember that the devil is a liar. Not everything that Screwtape says should be assumed to be true even from his own angle. I have made no attempt to identify any of the human beings mentioned in the letters; but I think it very unlikely that the portraits, say, of Fr. Spike or the patient’s mother, are wholly just. There is wishful thinking in Hell as well as on Earth.”