Great studies! How long did each take you? Economy of brush work is so difficult. It takes a ton of focus and knowledge of form. The artists that I've studied for this has been Joaquín Sorolla and John Singer Sargent. But if you want earlier I would go with Franz Hals as well. The essential improvement I can see with your work is getting a deeper understanding of form. What I mean by this is knowing in your head exactly how the body of the horse flows through space so well that you could rotate the horse in any angle and draw it. Of course you don't have to be that good... Like Kim Jung Gi.... but a better understanding will help you render the light on the forms. I point this out because most of the forms in these painting appear flat. They don't have the full illusion of depth they could have. If you want to improve your perception of form I would suggest Drawabox.com lessons. Marco Bucci has a wonderful course called Understanding and Painting the Head that is great as well. And of course Proko manichenization. (which I never spell right) Hope that helps! Keep up all the hard work!!! Very inspiring, I need to paint more!