[Zigûr]

I don't think he had enough power. The spiritual power of all Ainur was limited, but compared to that of, say, Morgoth, who bred the dragons and the werewolves, Sauron's power was miniscule.

Morgoth invested a great deal of his native power in his "superweapons" - the dragons in particular. Sauron's power was great, but not great enough to breed such creatures. Perhaps he might have come to control one, or bargain with one for alliance, as Gandalf believed he intended to do with Smaug, but such power as he had he needed to control his armies. This was sufficient, because by the Third Age especially his victory was inevitable due to sheer weight of numbers, both of Orcs and of the fallen Men enslaved to him.

I think we can see Sauron's awareness of the limitations of his power in the fact that his policies towards both Elves and Men in the West began with subterfuge, involving the Rings, rather than main force.

We might see the Ringwraiths as Sauron's equivalent of servants like dragons and werewolves, although their position was obviously more comparable to that of the Balrogs.