there's a lot to like about this drawing. I like your use of some very darks, here and there, and the composition is very good, very strong with the diagonal line being a clear focal point.
The placement of the eyes is anatomically accurate, something a lot of folks screw up. Yours are in perfect alignment with each other, same as the nostril, and these two pairs parallel each other, as they do in real life.
You have the multiple curves of the ears well done , particularly the left, which has a delicate shape (I adore horses' ears!).
However, the porportions are not as accurate as the details. The ears are kind of short, and the distance from the eyes to the very front of the nose a tiny bit long. If you were to enlarge the ears alone, this would help balance the overall horse.
Lastly, if you had a move obvious lightsource, this would help "round" the horse out. Even the halter , as it goes over the nose, can be shaded to show roundness, but you have to determine from which direction the light is coming.
The clarity of the tack is lovely, but is so much crisper than the horse that it almost become more interesting to look at than him. However, with such lovely eyes, I will always find myself looking there.