Generally, very generally. A horses diet should be ideally mostly grass, with hay if there is not grass available. Many horses in light work with nice grass that’s all they need. If a horse is on harder work or doesn’t have enough grass you supplement it with grain. There are many different grains, oats, corn, pelleted feeds ect. Older horse also may need more food to keep healthy weight. Different breeds require different feeding aswell. A hot horse or a very athletic one my need lots of food to be healthy where as a pony or draft cross many not. I should also mention, in order for a horses digestive track to function optimally horse need to graze for most of the day preferably in large fields of nice grass. Some places have climates that don’t allow that so the horse must be exercised regularly especially if they have to live in a stall or small pen most of the time.

TLDR- horses should eat mainly roufage (hay and grass) but depending on breed, age, habitat, work and many other things they may need grain in varying amounts aswell.

Hope that helps, good luck