The Little White Horse by Elizabeth Goudge

Monsieur Cocq de Noir, Maria, and Wrolf in the pine-woods

. . . And then it seemed as though the light were taking form.
  It was still light, but within the light there were shapes moving that were made of yet brighter light; and the shapes were those of hundreds of galloping white horses with flowing manes and poised curved necks like the necks of the chessmen in the parlour, and bodies whose speed was the speed of light and whose substance seemed no more solid than that of a rainbow; and yet one could see their outline clear-cut against the night-dark background of the trees . . . They were the sea-horses galloping inland, as Old Parson had told Maria that they did, in that joyful earth-scamper of theirs that ushered in the dawn.