Most rice bran supplements are at least 20% fat so how are they just a filler
Beet pulp is 1300Kcal per pound and great for a horse that needs weight or more fiber
I posted some really good TRUE fact links above that are very educational about the myths and realities of beet pulp ...
Per lb. (dry) beet pulp has about 1000 calories-sometimes much less. I know mine at home has 1020 per lb. It also has absolutely no value for vitamins, with the exception of fiber (where its quite good actually, at about 20%) In addition to that, it has a relatively low protein level (between 7-8%) if you want weight gain, you are much better looking for something that is in the 1500-2000cal intake and something in the (12-14%)
(scroll down to almost the last page for a comparison of calories)
The problem with rice bran is that it has a high degree of phosophorus per lb. From a nutritional standpoint, its much healthier than beat pulp, in terms of fat content as well as vitamins, however the extremely high phosphorous levels makes calcium absorbtion harder. (and for that reason, you might want to consider rice bran oil, as it generally has a lower phospherous level)
The point is that beet pulp or rice bran should not JUST be used. If you want to use it in addition to something, then that's the way to go. If you are using it completely by itself, the horse
Also, if you have the opportunity to, see if you can split his meals into 2-3 servings instead of feeding all at once. Then he will be able to digest more and use the calories and fiber more beneficially.