Guys, as for the whole multi-headed dragon deal, I don't believe that it's necessarily the Devil himself. The "seven heads, ten horns, ten crowns, and upon his heads the name of blasphemy" description is supposed to be symbolic of the government that the Antichrist ushers in. If you already knew that, ignore the last sentence. Not that Satan couldn't have transformed into a form that fits that description, I suppose.
As for the concept art, I think it was on NeoGAF or GameFAQs that it was revealed by some source that Dave Cox had said that the supposed "dragon form" of Satan in the LOS2 artbook was supposed to be an early concept for the Leviathan.