Very good start!
I'd like to see it once you have put in some deeper tones to add more detail. You need to bring out the cheek bone, running just below the eye, to add some more dimension to the face. Horses have a very prominant cheek bone, so that is an important part to add to a drawing.
You could also carry the line of the jaw up higher, nearly to the ear. At the moment, there is not enough definition between the jaw and the neck.
Above the eye, try adding the 'hollow' in the horse's skull. This will make the eye pop out a little more. Also, a little more detail around the eye will make it appear more realistic. The eyelid, some eyelashes etc. The corners of the eyes are often drawn down into vein lines a little further down the face and around the eye.
Hope this helps :)