In all honesty, I think that you should take a picture of a moving horse that you like along with the picture you've drawn, and find a tattoo artist who as done animal-type work before. Ask them to turn what you have into a more realistically proportioned line drawing. A good artist will give you one or two options (maybe more!) and transfer a mock-up of the tattoo on to your arm so that you can critique it and make it "just so" before they start inking!
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: