Of what nature is Satan?

Recently, a good friend of mine posed the question: > What (or who) is Satan and how do we know? I answered him, saying that Satan is an evil angel because this is what I've been taught since I was a child. However, after further investigation (meaning looking through relevant portions of the Bible) I could not justify my claim. In fact, the only form that Satan is described as having, in my investigation, is the form of a dragon. So, that's my question: What is the form of Satan? Is he/it a dragon, human, angel, something else, or is his/its form indeterminate? Please use scripture to support your answer, and thank you.