Could Maiar ever take non-humanoid form?
All 3 forms that I'm aware of were human/humanoid (5 Council Wizards, Sauron, and Balrogs).
Now Sauron knew well, as did all in that land, the fate that was decreed for the hound of Valinor, and it came into his thought that he himself would accomplish it. Therefore he took upon himself the form of a werewolf...
Then Sauron shifted shape, from wolf to serpent, and from monster to his own accustomed form;
from "Of Beren and Luthien" in "The Silmarillion".