Creatures of Tolkien's Imagination

I truly appreciate tolkien for the creation of beautifully crafted books, Filled with enchanting lands, strange creatures, and beautiful wordplay. The question that comes to me (and that I do not have time to research with a 3 year old and day job) Is -

How many of these lands and creatures are completely products of Tolkien's imagination. Or to put it in other words - was he the first to think up elves, orcs, and the balrog or were these mysterious creatures lurking in the minds of other men before him.

Either way he is the man who created the place for them in my mind's eye some 20 years ago.
Thanks for any insights or websites - I should like to read other works by early authors of middle earth type lore.