To be blunt: Your understanding of volumes. It's easier to see a pony, flat as she is, and not see the volumes that make her up. Getting to know the spheres and cylinders that make up a pony will help you to be able to create your own pony poses.
....i DO see the cylinders and such, it's more that if i want a certain pose, taking 3 different pictures and drawing it myself OR take a picture i can draw it directly, instead of following each and every single step with cilinders.
The thing is that their forelegs seem to change joint orientation depending on how anthropomorphic they need to be for a given scene. Sometimes they look and move more like a real horse's forelegs, and sometimes they look and move like human arms.