I believe this was part of an anthology. I read it in high school during the late 80s but as it was at my grandmother's house it could have been written much earlier.
The story starts with a breeder of horses, I believe down on his luck without having a winning racer bred in many years. One of his mares gives birth to a horse that's clearly different. I think it was described as looking somewhat like a race car - sloping face, huge nostrils to suck in more air, heavily muscled, shorter front legs to make it more aerodynamic. He assumes it's sickly as it's smaller, iirc, but quickly realizes it's incredibly fast. Eventually it's entered in horse races and dominates the competition. There was likely some conflict here as other owners tried to sabotage it, I don't remember.
The final scene is the horse retiring to a field, done with racing, but the breeding looking into another field to see many colts with similar features having been sired by the mutant race horse.