As a non-hippologist (and certainly not a hippo-apologist), I may have it wrong. But it seems to be a sign warning that a horse that is about to poop is going to kick you.
But in the overall scheme of things down under all, with the enormous swarms of spiders, mice, pugilistic animals with built-in pockets, STD carrying non-bears, people who seem to not be clear on what is a knife, and unfortunately (but not at all surprisingly) deceased beloved animal nudniks, it is not totally unexpected that this particular scenario of caution has heretofore been relatively unheralded.
Just drive carefully on the wrong side of your upside-down roads, hang on tight, and steer clear of the prancing daguerreotype silhouette carousel creatures, if you aren’t too distracted by avoiding baby-eating dingos and alligators. I do need a new pair of shoes I can’t afford, should your car or stomping hooves cause an untimely demise of an ostrich, snake, gator, or other podiatric opportunity.