When creating portraits of animals, expression is what sets your art apart from that of another artist. In creating expression, the eyes are everything!
Body language is important as well. Notice the proud, upright stance of this lovely horse? The head carriage is typical of a Saddlebred.
This closeup of the head of the same image shows the details of the eye and it's individual expression.
The tack (or lack of) always plays a part in the final portrait, so it's important to depict the accuracy for the breed and discipline.
Artists need to know and study the horse, the different disciplines, the different breeds! They are not generic and knowing the traits of the one you are creating allows you to develop a painting far beyond just a photographic likeness!