@Background Pony #CC05  
That’s probably closer to show style. I completely glossed over that. Sorry!

@Shelltoon  
I tried the two circles touching, but that ended up making the barrel too long, so I overlapped them.

The improvement is most certainly noticeable! I think right now, aside from proportions, is to make things a bit more dynamic. Right now, her body is one of those rounded rectangles. Pony bodies tend to look more like a bean.
 
To achieve that, imagine the body as two circles stuck to one another. Then add some arcs. They should look like Us but stretched out from the edges of the circle. The bottom one will attach to the outer edges, while the top one will attach to the inward edges. The head should line up with the front of this bean.
 
As for legs, they should have a bit more curve. Imagine if you took a cylinder, and squished it at the top. The top part will be smaller than the bottom. Now all you need to do is add some curvature to it, and you have a pony leg.
 
Apply these tips, and you’ll see a noticeable difference in your pony!