Design a simple background where you will set up the trajectory of your poses.
In this exercise, the main difficulty will be to keep the volume and construction in the space.
The horse and rider are connected and move in the background, size and perspective will change according to the position and movement.
Try to keep the proportions in between the 2 characters, and mind about reduction in the perspective.
The position according to the horizon line will also influence the construction.
Stay rough on the first set up, then you can add more details.
Follow this link for 3D references
The following images are references collected around the web to help to understand the structure of the horse.