Checkout this study on what the transgression of Satan was that caused his fall, what the origination of demons is, what the serpent looked like, and the full story of fallen angels from beginning to end, read the full study here,
http://www.wisdomofgod.co/2018/09/02/the-dragon-the-origination-of-demons-what-the-fall-of-satan-was-and-the-story-of-fallen-angels/