If you're looking for originality in D&D you're not going to have a whole lot of luck except with aberations.

Almost every creature of any other type has its origins in some mythological body or other. Drow for instance, hail from norse mythology.

Even aberations are generally a nod toward the works of H.P. Lovercraft and his circle of author-friends, though none of the actual creatures of the type seem to be direct ports from that mythos.