Answer:
Black we are and much admired. Many seek us if they are tired. We tire the horse, and comfort man, and turn White when we've fulfilled your plan.
I'm not man's best friend, I'm their enemy, I can mark your end, yet you do not see me, I am very small, but very tough, If you have me, then you have it rough.
I climb higher when hot, But beware my rage, I’ll kill with a breath, If I can escape my cage
It floats over the land, It cuts the tallest mountain, Its voice is like a fountain, Its body like a snake, Will flow into a lake.
I have rivers without water. Forests without trees. Mountains without rocks. Towns without houses.
If it’s information you seek, come and see me. If it’s pairs of letters you need, I have consecutively three. What am I?