See this does not say Satan is an angel.  It says he is TRANSFORMED into an angel of light.  This means he is not really one, but he fakes it.

He is a angel of darkness but able to show himself as a angel of light

So my question is this: explain how this 'angel' is a dragon?  How is he a serpent?

 Satan has the ability to show himself to anything he like even try to show himself as Jesus Christ's so to deceive the whole world.

And God allowed him to do so even to proclaim miracles and wonders. So he will deceive if possible even the very elect all who are not protected by the Holy Spirit.