As you boys well know it is the rider that has some sort of impairment, not the horse and yes the horses and riders are drug tested as per any regular Olympic/competition standard. :)
Just wondering if it is the riders or the horses that have to have the impairment.
Watching a paraplegic rider would not be much more spectacular than watching an able bodied rider but watching a horse with three legs trying to jump - now THAT I'd watch.