Answer:
Two horses, swiftest traveling, harnessed in a pair, and grazing ever in places. Distant from them.
Goes over all the hills and hollows. Bites hard, but never swallows.
You use a knife to slice my head. And weep beside me when I am dead.
White bird, featherless, flying out of paradise. Flying over sea and land. Dying in my hand.
Each of these ends in a kettle full of precious metal and the double variety is quite awesome
What is always coming but never arrives?