Sorry I havent drawn horses in a long time as well as I havent drawn on Oekaki in forever, but here's my tips!
1. the head- I tried to show in blue off to the side how I usually sketch out a horses head. It has a large cheek bone, making it very rounded at the bottom. As well as if you want to go for realistic, the bottom of a horses head isn't straight either. They have a bit of a curved line again connecting to the muzzle, this is because of the horses tongue. But for the most part you got it pretty good considering I see people draw horses with wolf like head shapes.
2. Body tip that always helped me! 3 circle rule! 3 same sized circles, 1 for the chest and shoulders, 1 for the stomach/ sides, and 1 for the hindquarters! :)
3. The one thing that I noticed for sure is your leg anatomy resembles a paw'd animal rather than a horse. Horses legs don't curve so outwardly, they are more straight. Same goes for the hooves. (my hooves are bigger than they should be but that's just my style and I can't seem to do it right on oekaki) Horses legs can be SUPER hard so I would reference from pictures! I know everyone feels like if they have to use a reference then they aren't goof enough to do it from memory- IT IS NOT TRUE!
Pros of referencing from a picture:
-anatomy in your art improves
-you learn as you draw and look at real pictures
-and you develop a better understanding of how certain legs move and bend. :)
- ps, all professional artists need a reference at some point and it makes you look more crediable if you refernce
(just don't heavily reference or trace, and always be sure to credit your references, makes you look even better as an artist!!!)
Other than those tips, just need to make the ears smaller, body longer, legs a lot longer as well unless you are drawing a miniature pony, and the base of the tail should come up higher since it is apart of the spine, the spine doesn't just suddenly drop below it's actual butt :x
Also I know I have circles in the hind quarters, with lines connecting them, thats just another tip I use myself when drawing horse legs, also helps me make the hindquarters as big and rounded as they need to be, They aren't random, thats just where the real skeleton kinda lays underneath.
Here's a picture off of google :)