This is the base model I used from Daniar Joldoshbek (artstation.com)

I modified the pose to make it match with the client requirements

Once the pose was done I proceeded to create a base model in Daz Studio 

I imported the horse into Daz 3d and started to play arround with the pose controls

I saved both the A pose and the Horse Riding pose to import them later into Clo3d/Marvelous Designer

I added the clothing using the A pose character so it´s easier to design and fit.

When the clothing is ready I transition the pose from A pose to the Riding Horse pose.

Once the clothing is done I import everything into Zbrush annd start to sculpt All the details.