If you do highlighting, you want to keep in mind a consistant direction of light. So, for example, it looks like the light might be coming from the upper left? Not sure. So, set the light source, then think which parts of the horse would catch and reflect more light , thus have a highlight, and which parts would then have a shadow.
And personally, the dark is too blueish for me. I mean the tail and legs.
I hope that's not too blunt of a critique. I am not a digital artist.
No critique is ever to harsh tiny! I wanted the points to be super super dark. However. I am using GIMP. And I couldn't see my lines when I used black, so therefore I wouldn't know where to erase to. It was very frustrating lol so hence the grey color
as for my lightsource I was trying to just get it from above, but I am still learning to highlight :) and thank you!