i hope this explains it!
i don't know much about horse anatomy since i've only been working with them for a few years,
but i really hope this explains a little bit of the kind of help you were looking for!!
(excuse any mistakes or badly-explained points...)
the ears
yours are quite "jelly-looking" at the top, and rounded out and thin all the way, whereas a "realistic" look for ears would be a slight triangle shape that flattens out more "slowly;" it could be mapped out as a triangle, but with the actual shape underneath. think of it as some sort of bullet or missile stuck to the top of their heads, to help them hear, i suppose?
the hair
your way of doing hair is quite alright as it shows the clumps of it, but realistic hair is more "separated" and you can tell apart the different groups of strands - try doing it like in the picture, or observing images of horses' manes to see how parts of their hair clump together to form the grass-looking edges as the mane flows down
the nose and skull
generally, horse skulls are quite thick, in width and length, and while your drawing of it was very good, the nose would need to be thicker, as they are naturally - they are also large animals, so their features will be enhanced (e.g. bigger eyes, bigger nostrils...) so make sure to reference horse heads or skulls when drawing - it tends to help a lot!
the mouth
horses tend to have lips similar to us, too - it is quite visible from the side that some horses have a "3"-shaped mouth (or, a 3 pushed onto its side..) and it definitely helps to imagine a ": 3" face when drawing its mouth, as it applies to front-facing views as well for some horses!
the cheek
think of it as a squashed circle with sharp corners - rather than drawing a very large circle, try drawing a square to map it out, and then doing a very slow, shallow, curved line, like a graph of some sort (if that helps?)
the muscles
it is better to add some enhanced muscles to horses when you draw them, as sometimes these are also visible in real life! if you get up close and personal with a horse, particularly when grooming it, if you are observant you will notice that there are parts of the horse that "go in," or even stick out a little - one of the main features that are the most noticeable on the front of the horse is its elbow muscles and neck muscles - it will be worth drawing these sometimes, as they are some of the most obvious when it comes to what muscles you can see on its body!
this also applies to the legs, as they are also quite thick because of the muscles and bones! keep note that there are also muscles and bones and blood vessels (that i forgot to mention but are worth noting, namely on the legs and face!) that should be drawn, as they are also visible and also make up some of the "space" on a horse's leg, minus the fat and skin there - try to keep the legs slightly thick, or from becoming four twigs!
the joints
the joints are another thing - i had so much trouble with these when i first started drawing horses! the knee is very round, or it's actually rather squared out in real life, but if you look at it and reference from photos and such, it mostly is just an elongated, squared-out circle, and when mapping out the horse when sketching it, it is definitely worth drawing circles at the joints - the upper forelegs and lower forelegs are also round-about the same length, on most horses anyway, so try and keep this in mind when plotting their legs for difficult poses such as jumping or galloping horses!
