Get a Horse!
This is the first Svetz story. Terrified time-traveller Hanville Svetz is sent back to the pre-atomic age to capture a horseāa simple-sounding task, until Svetz' horse turns out to be....!
First Svetz story, renamed "Flight of the Horse."
Anthology, The Best From Fantasy and Science Fiction, 19th series, edited by Edward L. Ferman; The Saturday Evening Post, June-July 1974; Graphic, Questar, Golden Press; Graphic, Starstream, edited by Roger Elwood; Anthology, Unicorns!, ed. Jack M. Dann & Gardner R. Dozois, Ace 1982; Anthology, Roger Caras' Treasury of Great Horse Stories, ed. Roger Caras and Martin Greenberg, Galahad, 1993; Anthology, Unnatural Creatures, ed. Neil Gaiman, Harper, 2013; Flight of the Horse, The; Rainbow Mars; Best of Larry Niven, The