Werewolves, Vampires and other creatures?

[Puddleglum]

Originally Posted by Inziladun

Gandalf seems to have thought Smaug would have acted for Sauron, though whether he would have done so willingly is up in the air. In fact, that was given in Unfinished Tales as one of Gandalf's prime motives in helping Thorin retake Erebor.

It may be just your wording (if so, my apologies in advance) but I think Gandalf's statement was nuanced a bit differently when he said, in UT, "The dragon, Sauron might use with terrible effect."
"Would have acted for Sauron" implies, to me, that Smaug would have been concious of acting to aid Sauron (whether willingly or not). From Gandalf's word choice, there is an additional possibility that Sauron might manage to manipulate Smaug to actions that aid him without Smaug even realizing (or caring) that he is doing so.

'Course, if by "acted for Sauron" you merely meant "acted in ways that happened to benefit Sauron" (as opposed to "acted conciously for the benefit of Sauron") then I'd agree with you, but the wording "seemed" to not include unintentional (even unknowing) support as a result of being manipulated.