If you have a hyper horse, you might want to look at your normal feed and turnout routine before turning to a supplement. Most commercially prepared feeds (sweet or pelleted) will make some horses "hot". Some horses can also have a sensitivity to Alfalfa that makes them hyper.
The calmest diet for a horse is grass hay or mixed grass hay with no grain. If you have to feed something extra, use a ration balancer like Purina's Enrich 32 or Triple Crown's 30% Supplement. You only feed 1lb a day for full nutrition. If you're feeding adaquate hay (2.5-3.5% of their body weight a day, 25-35 lbs for a 1,000 lb horse) and you still need more calories, then you can add plain whole oats, or beet pulp, or alfalfa pellets. Beet pulp is the "calmest" of the three. A fat source can be good too, like corn oil, flax seeds, or stabilized rice bran. Rice bran and Flax are the "calmest" of the three.
Give the diet change a month and see how the horse changes (switch over to the new feed/hay slowly, over a 2 week period). If it doesn't help, then you might look into supplementing with Magnesium and B vitamins. Horses deficient in one or the other can be more exitable. HorseTech.com--Horse Nutrition Supplements--Equine Nutrition on the Net
is a great source for custom supplements. Their Glazen product is flax meal based and high in B vitamins. They will add Magnesium to it for you for just the cost of the added product. They do not charge for the custom blending. If you call and describe your horse's diet and issues, they can make recommendations.
Turn out is also VERY important! You want your horse turned out as much as possible, preferably 24/7 with only stall time to eat any feed. Turn out with friends/playmates is also important. Your horse needs "recess" time to run and play. Keeping a horse stalled is like keeping a 3yr old cooped up in his room with no toys. When you do let him out, you're going to have your hands full!!! ;)