Don’t assume stretching is always good for your horse! Depends on how it’s done, when it’s done, and how often, just to name a few obvious factors. (And yes, there are potential benefits to stretching, just some potential pitfalls as well.) In my book I teach a good alternative to stretching for the limbs, which is to use a combination of jostles and circles to help muscles relax and stimulate joint fluid.
I also believe that massage provides a much more efficient and effective way to improve range of motion. It can melt adhesions that may be restricting joint movements, and can also lengthen the collagen fibers (and their orientation to each other) within the connective tissue. Theoretically this is what stretching does also (one of the benefits), but only if the stretches are held for a length of time. (Some say at least 90 seconds for this to happen. Try telling your horse that as you’re trying to hold their leg in a stretch!)
So here are two links, one to the research article itself (the abstract and access to more), and another link offering a good summary of the research.
All About Equine Massage 