Here is a very interesting article on horses being used for search and rescue. It describes a demonstration using two horses. The demo proved that "a wide variety of horses can be trained for cadaver area/air scent locating" and also that horses can distinguish human cadaver scent from the scent of other dead animals.
The advantage of horses over dogs is that the horses can cover a much larger area in a much shorter time. Also, they seem to need less training.
Read the article . . .