I guess horse owners would love good photographs of their pride and joy, just as car owners pay for a photoshoot of their car.
This shot and the last horse shot are good. I don't know what classifies as fine art photography, but these are definitely good.
So the shots that are selling at the moment are shots of your horse, taken on your own time?
There'll probably be 2 parts to your products and pricing;
1) a full paid for shoot for the horse owner including prints
2) prints of your photos of various horses - but if these were set-up horse portraits (like the ones you've done) ie not candid, you technically would need permission from the horse owner to sell photos of their horse.
*I'm assuming that the law is essentially the same as for people; take a candid shot on public property of anyone and you can post it where you like and make money, but if you take a set-up portrait of someone (like I've done on the street) you need their permission to post it anywhere and you need written consent (aka model release) if you want to make money off it.
I'm sure there are horse owners who'd be a bit miffed if you took photos of their horse and then sold them without asking.
So I would make sure if you do a full shoot for a horse owner, that you a) make sure you retain copyright of the images and b) make sure they agree to you selling the prints (you could explain that they are only sold to horse enthusiasts not to the media) and get it on a signed agreement so they can't back out.
It's great when your hobby starts paying something back! I 2nd shot a wedding at the weekend and got paid for it. New equipment time!