...Is not a simple matter. Everything you choose to paint has to function as a design element, as a piece of a harmonious whole.
Some things are easy to incorporate in a composition. A house is a rectangle, a mountain is a triangle, a hill is a breast; all simple, pleasing forms. A mass of trees or a cloud share the happy facility of being pretty much any shape you choose.
But a horse has uncompromising facts of anatomy which must be accommodated without upsetting the balance of the picture. As a living thing, it will draw the eye no matter where you put it, even if it's only incidental to the main focal point of the painting.
And it is a collection of shapes: the truncated triangles of head, and neck, the sagging barrel of the body, and last, and most awkwardly, the legs. Many, many legs. Put those legs against a plain ground, like a grassy field, and they will divide it into beautiful, interesting paper cut outs, with nuanced straight lines, graceful curves, and odd little sharp angles. Which might be bad, because these shapes are too interesting, and could take attention away from where you want it to go.
One solution is to incorporate the horse into a tonal mass so that it doesn't stand out too much, blending into the background a little.
Paint the horses lying down, and you either have rounded boulders that fit in easily, or a sprawling mess with legs pointing everywhere. Painting is about as honest as stock photography, in that you're always looking for the best angle to show off your subject, and trying to avoid the awkward views.
If you want to see what to do with horses' legs, check out Uccello's 'The battle of San Romano'.