There are many things that people feed their horses. Here is a list of some stuff and notes.
1. Bran Mash (Good for ill or tired horses/ponies)
2. Hay cubes/pellets (condenced grass)
3. Crushed/Rolled barley (Good fof winter feed)
4. Linseed (Makes coat shine, but is poisonous until it has been cooked.)
5. Suger Beet (Given to a horse that needs to be fattened up. It must be soaked in water overnight first.)
6. Oats (Given in small amounts because it gives a horse too much energy, and make it exitable.)
7 .You can also give your horse a salt lick for extra salt that they lose when they sweat.
8. Stabled horses are sometimes given chopped-up fruits and root vegetables to eat. Make it small enough so your horse dosn't choke.
Carrots
Apples
Rutabaga
Those are some that you can feed your horse.
Also horses eat alphalfa hay, it a hay net or holder. Can be found in feed stores.

There is a book that has all of this information and much more in it:
Superguides Riding bye Lucinda Green (Can be found at www.amazon.com and www.bn.com
Hope this helps!
Dogz