You can use a log to keep track of all sorts of information, depending on what your goal is. For example, here at Meredith Manor, we start students right off keeping a log of their training or riding time that they'll carry with them when they graduate. These logs are modeled on the kind of logs that airplane pilots keep. Students track how many hours they worked on different kinds of horses and under what kind of circumstances. The logs are tangible proof of their progress toward their goals of becoming professional horse trainers or riding instructors.
Amateur riders can keep a workout log on their individual horse.
Just like someone trying to build their fitness level might keep track of how many miles they jogged or how many times they were able to lift so many pounds, a workout log is an excellent way to plot a horse's progress. As you review the horse's log after several months, it might begin to show a pattern that you missed as you went from day to day. On those days when nothing seems to be going right, you can look back in the log to remind yourself that even though the horse seemed to be going backwards today, he really has been going forward since last week or last month. A log can be as simple or as fancy as suits you. It really doesn't matter if you use a plain old spiral notebook or a fancy diary with a ribbon marker.