Recent research in the U.K. has shown that people with mental issues like depression, anxiety and dementia all showed significant signs of lower blood pressure and relaxation when they were petting dogs. It raised the endorphins naturally.
Having worked with Riding for the Disabled I have seen how horses can have the same/similar effect. The other thing about a physically disabled person riding was summed up for me many years ago by a wheelchair bound lad. He said, "When riding it is so nice to be looking down on people and not having to always look up."