There are a few considerations you should take into account when deciding on the right aperture to use for your image.
Firstly there is a big difference in capturing images of a horse and rider where you need to keep both in focus. There can be nothing worse than setting up a great image to find you have used too small an aperture number and have either the horses head in focus but the rider is out of focus or the other way round.
If the horse and rider are straight on then you need to consider the distance between the horses head and the rider and that this will require a larger aperture number to get them both in focus, where as if a rider is 45 – 90 degrees on you can use a smaller aperture number as you are only concerned with the width of the horse.
When starting out I would recommended a beginner uses and aperture of f/5.0 – f7.1 to make sure you have both horses and rider in focus. Once you are comfortable with this you can then play around with seeing what different aperture numbers offer you for your style of image.
Where as when you are capturing images of a horse at liberty as long as the horses eye is tack sharp or the first horse in a line then the rest is down to creative interpretation!
If you are capturing images of a horse at liberty then you can aim for an aperture of f/2.8-f/4.0 and as above when you are comfortable with this you can play with other settings to see what you like.