For sport you don't need it, its normally outside or in a floodlit arena, and many competitors, and spectators will greatly frown upon the use of flash, especially if it distracts the mounts, they are after all competing.
With "pet" shots again, I can't see many examples where you would have to use it. I'm a big fan of "natural is best" as far as lighting goes, and try to avoid flash where possible, most people would want nice pictures of their horses in bright daylight, its doubtful anyone's going to choose to have a picture took in a dingy stable that would warrant the use of flash, also flash on horse hair if not done extremely well tends to reflect horribly making the horse look sweaty/shiny.
If for some reason or preferance you want to use flash, check with the owner... and check with the horse!
Fire a few test flashs in a safe place to get the horse used to it, but remember you're trying to get it used to it, not bombard it! Safety first, a spooked horse can and will do a lot of damage to equipment, itself and people.
Always have the owner there, as they can read their horses better than you will be able to.
Sorry for the long post, good luck and remember to share the pics on here!
