The Wendigo by Algernon Blackwood

For a man of his years and inexperience, only a canny Scot, perhaps, grounded in common sense and established in logic, could have preserved even that measure of balance that this youth somehow or other did manage to preserve through the whole adventure. Otherwise, two things he presently noticed, while forging pluckily ahead, must have sent him headlong back to the comparative safety of his tent, instead of only making his hands close more tightly upon the rifle stock, while his heart, trained for the Wee Kirk, sent a wordless prayer winging its way to heaven. Both tracks, he saw, had undergone a change, and this change, so far as it concerned the footsteps of the man, was in some undecipherable manner--appalling.

It was in the bigger tracks he first noticed this, and for a long time he could not quite believe his eyes. Was it the blown leaves that produced odd effects of light and shade, or that the dry snow, drifting like finely ground rice about the edges, cast shadows and high lights? Or was it actually the fact that the great marks had become faintly colored? For round about the deep, plunging holes of the animal there now appeared a mysterious, reddish tinge that was more like an effect of light than of anything that dyed the substance of the snow itself. Every mark had it, and had it increasingly--this indistinct fiery tinge that painted a new touch of ghastliness into the picture.