ok, so hopefully you have editing ability so you can play around with the opacity of the layers to see for yourself, but if not I'll type in your name.
Notes on the redline:
what I did here was to draw basic skeletons (skulls especially) and then looked at the ref to draw a general outline. The horse on the right has such a difficult pose, I commend you for taking on a challenge like that. Sketching with skeleton and muscle anatomy is always the key for me with hard to figure out poses.
The biggest part of making everything fit together in these poses I found was the front half. The skulls helped with the proportion of the heads, and the muscles for the shoulders.
Being honest, all of the issues that outlined were fixed with bone and muscle placement. My redline version is certainly not perfect, but I think since by your gallery you seem to love drawing horses the way I do, bones and muscles are the thing to study. With that comes proportion and shading and all that.
Speaking of shading, I think your drawing could benefit from taking a few refs and going really intense with your shading. Darker darks, range of colors, specifically placed highlights. Notice how the shapes of the horse work with the shading. I can see that you did do some of that here, but as I've found with shading, there is almost always more to do with it.
Your drawing is really fun! I love the idea of these playful horses.
I'm sorry I didn't have as much to say as on the last redline. I think what I drew is fairly self explanatory, unlike my last redline. That said, if I didn't make sense or forgot to re-write any run-on sentences, let me know and I'll try to explain myself.