I agree that horses do not need grain *as a rule*. It is *generally* not great for them either. If it's unprocessed, it cannot be easily digested in the stomach & instead is digested in the hindgut, where it can cause colic, laminitis & other probs. Processed & well digested, it can still lead to probs such as insuline resistance from being too rich, especially if it's also sweetened.
Unnatural feeding regimes - in meals with many hours in between - is also a problem for horses who's systems are made for almost constant digestion of tiny quantities. Ulcers are one problem, but mass die-offs of the hindgut bacteria which needs constant feed of certain types can also cause reduced digestion & toxicity. Difficulty keeping weight on is one possible product of this. If you're going to feed 'hard feed', little & often - at least every 6 hrs or so - is important.