I don't understand how a person in charge of the stables could get away with starving the horse to such a visible degree. It is not only animal cruelty (which would trigger me if this was real) but most likely embezzlement. As in the stable hand probably sold the grain he didn't feed the horse for profit. Horses themselves are expensive property, buying, housing and feeding them isn't cheap. It's why farmers use oxen to plow, cause cows are multi purpose for food and labor and cheaper to raise. Just because the owner of the horse is the ill favored son, doesn't mean the family isn't paying you to do your damn job! Even if the stable hand didn't out right embezzle, his actions devalued the horse in his care, which like stealing money too.
Edit: The only way the stable hands behavior makes sense it if someone with authority ordered him to deliberately neglect the horse. Also, the Baron is just incompetent at managing his land and people. Not just the bandits, but the fact the people who work for him don't do their jobs.