At least for this other class of nature spirits, Goldberry is one as she is the
River's daughter. What these beings were isn't clear, they could be a
special class of Maiar, but I personally think not. They seem a bit more
'wild' and certainly often dangerous, even Goldberry in the first Tom Bombadil
poem, which I don't think Maiar would be. In a recent post I suggested
the spirit of the mountain Caradhras as another example. In the Medieval
world view there was a class of angels who didn't exactly join Satan when he
fell, but didn't exactly resist him either. They were more like
'neutrals' (read C.S.Lewis's 'Discarded Image' for more about them), who were
exiled to our world, where some were good, some bad, but most were 'dangerous'
to consort with. Tolkien in his profession would certainly have known of
the them, and maybe you could imagine Tolkien's nature spirits fulfilling a
similar role in his
subcreation.
