Answer:
Without me everyone would lose their head.
I have a straight back and sharp teeth to cut objects.
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 goes up and down, but does not move?
In a tunnel of darkness lies a beast or iron. It can only attack when pulled back. What is it?
I run through hills. I veer around mountains. I leap over rivers. And crawl through the forests. Step out your door to find me.