When you do a photoshoot with horses in action, you NEED to hold the trigger while the horse is running, so that you have a chance to get the perfect pose...
Horses are not like dogs or cats...
If you rely only on your eye/finger reflex to push the button at the right moment, it's already too late.
Also, when the horse is moving, you may think you have the perfect shot, only to realize on the computer that the focus is not right on the eye...
I also only have the kit lens and a 50 mm lens... so I rely on luck most of the times

I hear shooting horses is as hardas shooting a woman in bridezilla mode.