If horses were wishes, beggars will ride.

​It is a classic way of saying that wishing for something doesn’t make it happen. If simply wanting something were enough to make it real (like a horse appearing out of thin air just because you wished for one), then even the poorest people—who have nothing else—would have everything they desired.

It’s pointing out that if desires alone could create results, then even the poorest or most powerless people (the “beggars”) would have everything they want—symbolized by riding horses, which used to represent wealth or status.