Maiar Spirits' Bodily Forms

[Son of Númenor]

You should try reading the beginning of The Silmarillion.

Or the whole thing! It's great!

One more thing to add: the Ainur who were capable of changing 'forms' generally did
so based on their mood, & chose their usual forms based on their individual dispositions (hence the fact that some Ainur are 'female' & some are 'male' - based on qualities generally associated with one gender or the other).