Answer:
Always old, sometimes new. Never sad, sometimes blue. Never empty, sometimes full. Never pushes, always pulls.
A shimmering field that reaches far. Yet it has no tracks, And is crossed without paths.
You must keep this thing. Its loss will affect your brothers. For once yours is lost, it will soon be lost by others.
Two horses, swiftest traveling, harnessed in a pair, and grazing ever in places. Distant from them.
I can travel from there to here by disappearing, and here to there by reappearing.
People take me on walks and I run and have fun. What am I?