[Keeper of Dol Guldur]

It's just like with the werewolf servants Sauron as Thu had under his control. Morgoth took something viscious, but not evil (wolves) and introduced fell spirits into them. He did it to Draugluin, father of werewolves, and then tripled the power in one of Draugluins puppies, Carcaroth. Carcaroth was quadruple the size of Draugluin, who was triple the size of a wolf, and probably a little man-like. Morgoth may have simply taken reptiles and spliced them a little, then introduced pre-existing fell spirits into them (like Sauron did to the Barrow Wights). Who knows how many he had anyway.

I do think though, that the likes of Glaurung, Smaug or Ancalagon may have been some pretty horrific spirits as well. Who were they before they were stuck in dragon bodies? Eagles? Something along the lines of what the Nazgul rode during the war of the ring? I'd assume the latter. Anyway, noone said Tolkien discluded dinosauria from his tales. The Valar created much, and Morgoth destroyed it in his spite, constantly supplanting their efforts. Maybe he kept a few Tyrannosaur's around, and just did stuff to them. Lots and lots of stuff, until they were dragons.