Horses are incredibly sensetive beings. They experience their environments in ways we as humans could not even imagine. No matter whether it is positive, negative, excited or subdued hores soak up energy from around them. When we are working with horses they experience and manage every feeling and bit of energy that passes through our bodies. Reiki offers a way for both humans and other living things to release all the stored up energies we, or they have soaked up and held onto.
Here is a great article on why Reiki works.