After all, bread is, well, made of grains and oats, which horses eat. So, is it fine to feed your horse a slice of bread?
Yes...if you're being reasonable about it. Bread is rather rich (for a horse) and can make them put on weight if you feed them too much - especially if they're already having insulin resistance problems. (In fact, some people feed "poor keepers" bread because it IS so good at putting weight on them). Some horses don't like it, though.