Answer:
Two horses, swiftest traveling, harnessed in a pair, and grazing ever in places. Distant from them.
I can be written, I can be spoken, I can be exposed, I can be broken.
To unravel me you need a simple key, no key that was made by locksmith's hand. But a key that only I will understand.
What type of paper can you neither read nor write on?
What measures out time. Until in time all is smashed to it?
I never sleep yet sometimes I weep. I can get angry or just be happy. Wherever you go, I will follow. No matter what you do, I will be there. I cannot die. What am I?