Keeping in mind that I am self taught, and I do what works for me, what works for you might be completely different :) this is how I begin...
All of my OS (original sculpts) have been portraits of real horses. All of them have been alive and I have been able to visit them to get the appropriate measurements and photos. With Mistral however, he has passed after a freak accident :( therefore I am relying solely on photos the owners already have.
Luckily for me they were keen photographers and the horse widely campaigned so I have in excess of 120 photos of the boy. Most of them of him jumping.
By far the most important photo though, is this one;
And this is why...
This is the most 'square' photo I have of Mistral and from it I was able to get a fair idea of his measurements. Once I have found the horses points, where the significant joints are, I can build an armature to scale. Bone length does not change, joints open, close, flex and turn. Muscles squish, slide and stretch, but bone length stays the same.
The only lengths that will change from a static to a moving horse is the neck, back and ribs, lengthening and shortening to the horses motion.
Next comes the armature. For the smaller horses, such as Humphrey, I literally just twisted wire together. For the bigger horses, like Mistral I have used a plumbers T-join, a screw pipe and a flange.
I thread wire through the pipe and hold it using the wax clay, nothing permanent as I want to be able to reuse the join for the next horse. I then measure and mark the joints with a Sharpie, then bend them to the pose.
This was Whispers armature;
The next most important photo is the pose shot. Mistral flying.