BUT - that being said - when you associate the fact you are popping the horse with something they can't SEE (so they might think they've legitimately hit their head on something above them) then it becomes more logical. That also being said, the object you smack them with shouldn't matter, right? The sensation of something running down their head (egg, ketchup, water, etc) should not be relevant. Just the fact they've encountered a barrier above them that they hit if they rear should be enough.
Regardless, I don't think trickery is the right way to go about it. As I said before, find the root of the problem, pull it out, fix it. Otherwise you'll just be stuck temporarily fixing a dangerous problem over, and over, and over again.