Ditto on the guywires
I don't have the horse yet
but I have had amazing success with guywire and tent spikes for some really tall and even top-heavy props. One central pole shoved at least a foot into the dirt, and at least three lines, preferably four. Bamboo sticks spray painted black are great, too, if your prop is going to be up for a while (I usually leave mine out at least a month). Duct tape metal loops on the bottom of the bamboo poles to attach the tent spikes, and don't forget to angle the tent spikes w the bottom toward the prop, and the head away from the prop. This ONLY works if the prop is not in an area where people will be walking next to it, because it is a big trip hazard (I've tripped on my own a few times!)
I don't have the horse yet