Rice bran oil is what you get when you press rice bran- it (like all oils) is almost 100% pure fat. Fat is high energy, but it's considered a cool energy because it doesn't cause spikes of energy like the sugar/starch (NSCs) in grains and sweet feeds. Swapping out some (or all) of the grain in your horse's diet for lower NSC products (like rice bran, beet pulp, or alfalfa) should help level out his energy, and is healthier anyway, as horses' digestive systems don't handle NSCs very well to begin with. (Though as a side note, if you cut out all the fortified grain you may need to supplement with a multi-vitamin or ration balancer)
The downside of rice bran is it's omega omega-3 (an anti-inflammatory) to omega-6 (an inflammatory) balance, which is heavily skewed towards omega-6. This might not be an issue if your horse is on pasture and getting plenty of fresh grass (their primary source of omega-3), but could be a very big issue if he's on grain & hay only, especially if he has arthritis or other condition caused by inflammation. Rice bran is also naturally very high in phosphorous and can upset the calcium:phosphorous balance in a horse's diet depending on what else they're being fed (not generally an issue for horses on a high-alfalfa diet, for example). "Fortified" rice bran has added calcium to balance out the excess phosphorous. Rice bran also goes rancid VERY quickly (sometimes before it even makes it to the shelves in the feed store) so always
buy stabilized rice bran.
If you're interested in feeding oil, flaxseed/linseed oil is the healthiest; it is the only oil that has more omega-3 than omega-6 (well, after fish oil, but good luck getting your horse to eat that
). The next best commonly found oil is canola oil, which has about 1/2 has much omega-3 as omega-6.
I also prefer feeding flaxseed over rice bran (in their non-oil forms). While rice bran is about 22% fat, as I mentioned before, flaxseed is ~40% fat. You'll get some differing opinions about how to feed flaxseed- the cheapest way to feed is to buy whole seeds and either feed them whole or freshly ground before each feeding. Some people will say they won't digest the whole seeds, while some studies have shown that even if the seed shows up whole in the manure the nutrients are still extracted through the hull. I feed whole seeds to my horse and never see them in his manure, but I only feed 2 oz per day (he's an easy keeper). There are several stabilized ground flax products out there which are good alternatives if you feel it must be ground but can't do it at each feeding, but they're generally much more expensive than whole flax.
Sorry that turned into a novel