I'm not the best at explaining myself, but here goes..
Nice work. Excellent advice from Horsesdontlie! I agree with looking at the way horse's bodies are not always round. Your drawings would be amazing as cartoon drawings but now look at realistic horses. Take a look at their shapes, and you'll see a horse is not perfectly round.
Your shading is coming along. But now try not to make the outline of your horse darker than the shading, try to blend it together. Take a look at any 3D object - do you see an outline? Nope, it all blends into one. Don't be afraid to press on with your pencil in ALL areas of the horse and then blend it into the 'white' areas and outline
I used to do it, then I learned to blend my outline into the whole picture. Take a look at this I did years ago compared to the bottom one..