I tend to agree with you Kuruharan on the point about the Maiar. Dragons were far too intelligent and crafty, some, like Ancalagon, had a sort of evil power to them. This could not be achieved by breeding alone. A lizard could not just develop these abilities, no matter how big it got.
The origin may be two-fold. I believe a Maiar spirit was present in a "shell" of a dragon. Melkor could very well have bred lizards to an extreme size and then have a fire-demon enter into it. This would explain where the power of the dragon came from. Tolkien of course never really justified this, it is conjectural, but fairly reasonable I believe.
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