WildWillow said:

Hi!

T1 artist here. :)

Lighting goes hand in hand with shadows - both are all about looking where light falls in your artwork, and therefore, were shadows would be cast.

When putting a horse into a background, look at the background to figure out your light source. That will give you a guide as to where light would fall on your horse.

I took an example from your shop and drew some lines over it to help you get an idea. In this background, light is being cast from the upper right hand corner towards the lower left hand corner. You can tell this from where light and shadows are laying on the mountain.

I then drew over your horse to show in black, where I would place shadows, and in yellow, where I would place highlights. Notice how this is following the direction of where light is being cast in the background.

In red, I displayed the general direction of where the horse's actual shadow on the ground would be cast.

I know this is brief but I hope it helps you out. I'm happy to answer any additional questions you might have. Just send me a messager. :) I also have a Tutorial thread in the forums that might be of help to you.