Answer:
Two horses, swiftest traveling, harnessed in a pair, and grazing ever in places. Distant from them.
I appear in the morning. But am always there. You can never see me. Though I am everywhere. By night I am gone, though I sometimes never was. Nothing can defeat me. But I am easily gone.
This thing all things devours: Birds, beasts, trees, flowers; Gnaws iron, bites steel; Grinds hard stones to meal; Slays king, ruins town, And beats high mountains down.
Whoever makes it, tells it not. Whoever takes it, knows it not. Whoever knows it, wants it not. What is it?
What do you purposefully put lots of in and on your body, but run away from when you encounter it outside?
People take me on walks and I run and have fun. What am I?