Dr. Temple Grandin is an animal behavior specialist with the Department of Animal Science at Colorado State. She indicated in Equine Veterinarian that it is critically important to make a horse's first experience with anything new a positive experience. If a first experience at entering a trailer or entering a horse stock is frightening, it may be difficult to get the horse to go back in the same area. For this reason, it is important to go very slow when introducing horses to new experiences and after they have accomplished the goal, give them a food treat. Certainly if you are trying to introduce a horse to a new area, make sure the footing is good because if the horse slips and falls, it is very frightening and the horse will associate the area with fear. And these fearful memories last a long time, sometimes forever.
It has been shown that new objects presented to a horse can be non-frightening if the animal is allowed to approach the object voluntarily. Grandin gives the example of a large umbrella. If you pull out an umbrella it will startle the horse but if you place the umbrella in the arena and allow the horse to approach it naturally, it will do so and not be afraid. For example, if you have a horse that is fearful of flags, place flags around the perimeter of the arena and then let the horse approach them voluntarily rather than being surprised by them. A horse's memory is specific as he may be afraid of black hats but okay with white hats so the key is introduce all things slowly and give a treat when the task is accomplished.