If you Google 'wool duvets',you will find plenty of them. Wool would have been the fiber used until the first cotton mills were built in the late 1700's.But,I think cotton is still more common as it is most likely cheaper to produce,lighter weight,easier to care for,and is not a potential allergen/irritant as wool can be.
My quilt is filled with down. Most natural fibre quilts are filled with feathers or silk. Artificial fillings vary. Way back last century you might have found quilts stuffed with kapok. I have never heard of a quilt being stuffed with cotton, especially as it would soon flatten with use. Wool filling might require dry-cleaning instead of washing.
I have a Devon Duvet filled with sheep's wool......It wonderful to sleep under, cosy and not heavy. I could never go back to anything else now.
It's machine washable but not suitable for dry cleaning.