I think there may also be a breeding stock issue. I'm going by the words of my A-Z of Tolkein (which is not always all that accurate) but I seem to recall that the main difference between Wolves and Werewolves in ME is that wolves are basic biological creatures, whereas Werewolves are actually minor spirits of Arda (i.e. Maiar) brought over by Morgoth who took the forms of wolves in ME. Sauron was Commander of the Werewolves, but I'm not sure he was their creator (in the same way he could command Orcs, but did not make them.) In otherwords, to make a werewolf, you have to have Maiar under your command. And Sauron, a Maiar himself, doesn't, he just isn't that powerful. In all likelyhood by the third age Sauron would not be able to turn HIMSELF into a werewolf (like it used to be able to do). Also based on those other rare occasions where we see Maiar having children in ME (like Melian), thier offspring are not themselves Maiar, though they may have some "boost" due to thier Maiar blood. So in all likelyhood, the spawn of werewolves would be ordinary wolves, though possibly of greater than normal size and power (actuallty, since we don't here of them before the Third age, maybe that what Wargs are biolocally, the spawn and decendants of the spawn of the werewolves Sauron had under his command before.) Any werewolves Sauron still had in the Third Age would likely be ones who escaped the massacre on Tol-um-Gauroth or were not actually there when it occured (Maiar usually don't die of old age, so any werewolves who were not killed would likey keep going on) who would be few and sparse (which would explain why they play no large part in his attack plans.)
Dragons would have a similar problem. It is said that Smaug was the last of the great dragons. Who would one breed with him? A cold-drake hen from the ones that pestered the dwarves? Even going back further Great dragons seem to have become a little sparse post-Morgoth. Given how much creatures seem to have diminished by the Third Age, the possible dragon breeding stock was probably too weak to give a likelyhood of spawning extra Smaug's or even dragons close enough to him in power to be useful. Sauron was lucky to find the nest of Fell Beasts in the Mountains of the Moon (who, based on their description, could be some sort of dragon kin biologically) and as far as we know, he couldn't even get THEM to reproduce (or why he had such limited numbers, and was not fielding a full Mordorian Airforce.)
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