I’m curious on if you’ve ever experimented with wide small vs dynamic on the Z6 for shooting horses? I also like to do photos of horses (as I own one and end up spending a good amount of time at the barn) I’ve always used dynamic, but now you have me wondering if wide small would work better?
I don't shoot horses, but Eric's answer is probably the best. See my autofocus overview here for the differences:
The reason wide will generally work better is that the camera will lean toward foreground subjects and doesn't require a precise point.
Dynamic would work ok too, but you'd need to be more careful about where your AF point is when you first start focusing and in keeping that same point within the AF area box. I'd imagine this could be more difficult with the types of movements one would expect on horseback--namely there is both a cyclical vertical movement where the height depends only on distance while the frequency depends on speed; as well as a linear horizontal movement, which changes depending on relative direction of travel as well as distance. This probably too erratic to keep track of while ensuring that it all stays within the box (while risking blocked shot response to being too long to refresh focus if the horse is moving toward you at all).
To simplify all that, use wide area. Let the camera do the thinking. It will basically say: "within this box, what's the best thing to focus on?" And most of the time, it will be the rider and/or horse.