in reality a diet low in sugars and starches and higher in fats and of course based off of forage is best for ANY horse. Very few horses need the added sugars and starches from grains like Corn, Oats and barley.
So ideal diet for ANY horse is hay in approriate amount to keep horse at good weight
Vitamin/minerals to balance that hay
Fats as needed for energy and weight gain
I have been called the NSC Nazi more then once ... I hate traditional feed methods of loading our horses up on grains and junk food :)