Answer:
Large as a mountain, small as a pea, Endlessly swimming in a water-less sea.
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.
Two bodies have I, though both joined in one. The more still I stand, the quicker I run.
Pregnant every time you see her, yet she never will give birth.
I bind it and it walks. I loose it and it stops.
What never gets any wetter no matter how hard it rains?