Your horse may be old, he may suffer from Cushing’s disease (ask your vet if your horse needs to be tested), you may just have a hairy horse. But when it’s May 1 and all the other horses around are sleek and shiny looking and yours still looks like something from the set of 10,000 BC, you should probably body-clip him. I’m saying “him” because the horse I clipped today is a gelding. You don’t need to do it because other people and horses will judge you or your horse. You need to do it because your horse is sweltering day after day. You don’t care how he looks, you just don’t want him to suffer.
Maybe you are working your horse in the winter, in which case you may be preparing to show or do something else where other people do see your horse and judge you by his appearance. If this is the case, you should just pay a professional groom to do it. It takes a lot of time, equipment and skill to do it so your horse to looks good. It is probably worth paying someone else to do that if you don’t know how, which you don’t if you are unprepared. Don’t make the mistake of thinking it will grow out by show season. A bad clip job will last all year.