It needs to be SO much darker - depth and contrast is the only way you can get a lifelike, bold and pleasing to the eye portrait.
As you draw, block in the sections that are truely black - rim of the eye, pupil, in the ear, under the jaw, behind the jaw, nostrils etc etc etc.
Then figure out where the highlights are, leave them completely white - highlight in the eye, top of the cheek bone, tips of the ears, rim of the nostrils and shine along the rims of the eyes, and so on.
From there, you can start adding your tones in between.
Think of white as 0 and black as 10, all portrait work needs to have 0 and 10, and everything in between. You need to spread these tones equally, your current drawing shows a lot of 10 and 8-9s around the muzzle, but very little in the lower scale. While the rest of the head/neck is the opposite. Thus the viewers eye is immediately pulled to the muzzle which becomes highly distracting.
I am assuming the horse is grey? There is still an incredible amount of contrast in the white parts of a grey, which you can capture without losing the effect of a darker nose.
I suggest you go back and try to create some more depth in this piece, as the proportions are lovely so it would be a shame not to try and rework it.
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