I'm sure it took incredible talent and creativity to get those pictures, that's why it can be called art.
Is it art? Yes, all photography is technically art. But the idea here has nothing to do with art, it's about the interesting images which stand on their own without needing to resort to esoteric ideas or arguments.
But what do we get out of seeing horses from that perspective? I'd like to know what about those images fascinates you. Have you ever thought to yourself, "I'd really like to crawl under a horse to get a frontal view of its schlong and tuches"?
What do you get out of seeing yet another sunset picture? A funny video? The view out your window? You get a feeling of enjoyment, even fleeting, of seeing something pretty, funny, or interesting.
In this case, you get to see a very familiar thing from a totally new perspective. It's unique, humorous, and makes you think about maybe looking a little differently at everyday things.
Whether or not I ever thought of taking a picture of a horse's bottom is irrelevant (why do I feel that there is no good answer here
); what matters is the final pictures and if you like it or not.
So what we have here is a guy who invests a great amount of energy and skill to create art that nobody cares to see. Hence the comment art for art's sake.
You don't care to see ≠ nobody cares to see. You obviously don't find this interesting at all; that's fine. But there are a great many people who
do care to see.
Investing a great amount of energy and skill to create art for art's sake? The guy is a photographer and a businessman who makes a ton of money off this project - this is his fourth and most elaborate instalment in the series (after dogs, cats, and rabbits).
Art for art's sake? Hardly. This is good photography and good business.