Answer:
Brown I am and much admired; many horses have I tried; tire a horse and worry a man; tell me this riddle if you can.
What is as big as you are and yet does not weight anything?
Everyone has it and no one can lose it, what is it?
Vikings use this for warmth.
My teeth are sharp, my back is straight, to cut things up it is my fate.
I’m grown from darkness but shine with a pale light. Very round I am and always a lady's delight.