The abuse and deaths of animals in movies is nothing new, sad to say; in old movies, horses were often injured or killed through the use of tripwires. In modern flicks, horses are trained to fall safely, which is obvious when you know what to look for: the horse will pull its nose in towards its shoulder before keeling over. The American Humane Association has been monitoring animal welfare on sets since 1940. Still, accidents do happen; a horse was killed on the set of Flicka while another died rather bizarrely during filming in India.
To combat these tragedies, equine biomechanicist Dr. Sian E.M. Lawson is now working with directors to create realistic digital horses in movies such as Kingdom of Heaven (above), Alexander and King Arthur using digital capture methods; similar technology was used to create Gollum in the Lord of the Rings trilogy:
Motion capture allows movements to be filmed in controlled safe environments and to be linked together creating a realistic sequence that the horse didn’t actually perform. However it is not possible to provide the movement for sequences such as a horse falling or rearing unless the equine actor actually performs this. To solve this dilemma I have begun to create a very complicated biomechanical tool called the Digital Equine. This will allow horses and their movements to be completely computer simulated, replacing the role of horse motion capture in the film industry and even removing the need for real horses altogether. [...] To enable the highest quality of motion to be simulated Equine Mechanics’ Digital Equine model of the horse is pain-stakingly biomechanically correct including bone shapes, joints, tendons, muscles, ligaments, mass and inertial properties. It provides real motion as it effectively uses a virtual horse musculo-skeletal system. It can then be adjusted for breed and type and riders can then be added or not at the animators whim. (More)
Dr. Lawson has her own website with some great info on equine biomechanics.