There are many different testable hypothesis and experiments that can be made in order to prove this. Since there are no available statements let me provide one.
Testing and Comparing the excrement of wild and tamed horses.
This is because the excrement of tame horses will show higher levels of man-made chemicals and additives due to the different man-made horse food that the owners are giving them. Wild horses on the other hand will not since they nourish themselves on naturally grown grass.
