Okay here it goes..... stay with me if you can!
My first bit of advice is to not draw it yourself unless you are a experienced artist. I would suggest commissioning an artist to draw one for you. But if you really want to draw your own, here is my advice:
First I took your drawing and made a very rough sketch here:
I would suggest studying the musculature of the equine form, I suggest some simple bone and body anatomy reference studies: (De)Constructing the Arabian Head by *Ehetere on deviantART
/ Horse Skeleton by ~Tikall on deviantART
/ Equine Proportions by =ElaineSeleneStock on deviantART
/ Horse Muscles Anatomy by *DarkKenjie on deviantART
Deviantart.com is great tool.
Then combining your reference pages find a photography of a horse running, and carefully reconstruct the shapes within the image while following the general horse shape. Your can see how I have done this in my sketch some notable examples looking at my sketch, (which could be refined to a much higher degree but I kept is simple)are the neck and shoulder shapes, as well as the head attachment which is slightly lacking in your drawing. Also notice the rear legs, yours go straight, while a horses have a triangular hock.
But I put yours next to mine so you can see the differences yourself, Hope I helped a bit. I'd be glad to answer and questions.
For further reference of what I mean by refining the sketch which I think is what you need to do, here is fully refined (or fully fixed & edited lining) of my own:
This started as what I have exampled as a sketch for yours and I kept using smaller and more exact lines until it became acceptable to my own artistic vision.