It has been said that the Valar cannot create life itself, as only Eru is able to do so. considering this, how did Morgoth manage to create his Dragons? is there any solid explanation for their creation?
Morgoth warped serpents to create were-worms. Then I think he corrupted these into Glaurung was the first fire-breathing Dragon without wings and found them of great use then he increased their destruction by corrupting them again to have wings. Then in time came other dragons and Smaug is one of these. I have only guessed from my readings of Professor Tolkien, this is my answer.