It is always great to look at a
horse skeleton, and try and sketch that out first before fleshing out a sketch. It gives pretty much instantly a better grasp of anatomy.
Another thing you can do is to take a look at
this, it shows easily the horse anatomy and how to build up the horse body (you can drop out the centaur part)
Some keypoints that I spotted to point out are;
1) The withers aren't really that wide, as seen on the illustration it's like a little lump that sticks out.
2) The shoulder needs to be smaller, it starts sort of narrow on the withers then widens out (as seen on the illustration), and the stomach is more barrel shaped. The back also needs to be longer.
3) The neck in general is longer than pictured, even when curled. But you've got it put in place correctly though! Just a bit short
4) The back part of a horse is higher than the frontpart, so in a way it needs to be tilted upwards a bit as the hips are rather tall in a horse. This means also that the hocks should be higher placed than the knees.
5) The head is a bit small compared to the body
6) When placing the legs think of them as mirrors. If the left front leg is put in front of the right frontleg then the right hindleg will be in front/under the belly.
I made also a quick
redline to show off these points to hopefully make better sense of them. But I am by no means an expert on the anatomy field of equines, I am still learning myself. So take my words with a grain of salt, as you should do with all information in general. These points are just what I would've done differently.
Hope this was useful in any sort of way though!