Horse owners and trainers commonly ask veterinarians for medications to calm their horse for training or showing purposes. Veterinarians have to be careful about dispensing these drugs. You have to realize that after treating a horse with a sedative, the horse could become dangerous to the rider; if an accident happened, I am concerned the vet could be partially liable from having dispensed the drug.
There are some dietary supplements commercially available for horses and although there is almost no scientific information about the effectiveness of these products, they have been used for years. Some of the common ingredients have a basis in theory in other species. Dr. Sue McDonnell indicates that one example is l-tryptophan, which is a common ingredient in many commercial calming supplements as it is a precursor to the neurotransmitter serotonin. L-tryptophan has been shown to induce calm and fatigue-like behavior in several species, and at 1 to 2 grams twice daily it has a mild calming affect on stalled horses. Another available product is a pheromone called Modipher EQ. This product has been used in situations involving fear, stress, and anxiety, such as feet trimming or hauling.
The pheromone approach to behavior modification sounds like a really good idea. Unfortunately, the controlled blind studies at Dr. McDonnell's University of Penn Equine behavior lab have shown no effect of these pheromones, at least in horses. I have seen some effects in cats and dogs with pheromones, so more studies are needed.