There are many correct answers to this question, for horses eat several different things. One thing they eat is oats. They are great for horses because they have low energy value and high fiber content. Most owners give their horses naked or hull-less oats.
Along with oats, horses have a diet, which consists of grass. Some horse owners enjoy horse for their diet of grass because horses eat the grass down to the soil. Them doing so prevents the grass from growing and eliminates the need for the owner to cut any grass. Another food they eat is hay.