Robert F. Young 
The Forest of Unreason [EPUB ebook] 

Mortby saw the hoof-marks first. It was the morning Reinman had come up missing, and he and the four other remaining members of the expedition had set out into the Kontawaba Forest in search of him. The hoof-marks were on the opposite bank.
Robert F. Young was a Hugo nominated author known for his lyrical and sentimental prose. His work appeared in Amazing Stories, Fantastic Stories, Startling Stories, Playboy, The Saturday Evening Post, Collier’s, Galaxy Magazine, and Analog Science Fact & Fiction.