Find images of real horses (stock, I would suggest), and study how the light and shadows play on the muscles and features, and where the light source is coming from in each image.
From that, on your drawn images, I would suggest picking a direction of light source, and focus the shadow and highlights on this.
For highlight and shadow color, I would suggest darker colors of the coat version you are working with. So with a chestnut, a dark brown could be used for the shadows, and a light peach/tan for the highlights.
Swatches on the side of your peace always help so you consistently use the same colors
Morning Showers then Cooling
