Answer:
People buy me to eat but never eat me. What am I?
Face with a tree skin like the sea. A great beast I am. Yet vermin frightens me.
Tear me off and scratch my head what once red is now black.
A dragons tooth in a mortals hand I kill I maim I divide the land.
Though desert men once called me God today men call me mad. For I wag my tail when I am angry. And growl when I am glad.
Give it food and it will live, give it water and it will die.