Horses can certainly distinguish taste. They have taste receptors, similar to taste buds. In the horse, taste receptors are commonly located on the roof of the mouth and the rear portion of the tongue. Smell probably plays a larger role in horse food preference than taste alone, however, it is very difficult to separate smell from taste when studying food preferences (ideally all foods would have to have the same aroma but different tastes). I found no research indicating exactly how many taste receptors the horse has.