I work at a endurance barn, and she feeds beat pulp, comlete advantage, ultium, and oats. Then she has just regular I think its vegtable oil but im probably wrong ill check when I go back to work tonight, and then she feeds Cool cals wich is a fat thing. For the endurance horses the more fat the better, they need more fat so that they can switch over to the stored energy (fat).
This article explains the difference between aerobic and anaerboic activity. Whether you need fat or carbs depends solely on what activity you will be doing with your horse. It only takes a few minutes to sign up for the website and the articles are very useful.