For me, this is a grey area. I've noticed that PETA has guidelines that they want to use for "humane horse racing". so even they don't seem to want to abolish horse riding.
I think all working and companion animals fall into this grey area. I've gotten to know a few police dogs and I've always got the impression that going to work was fun for them. Oh, boy we get to ride in the car. And meet people. and sniff things. Training is like a game to them. and there are toys and treats! and naps.
There may be some anthropomorphizing on my part but people like to have a purpose, feel needed, accomplish things, get attention. I think animals might share some of those things too.
When i was a teen I had a summer job at a stable. Some of the horses were rentals. You rented a horse and went out for a ride. and some of the horses were tenants and their owners came by and rode them as often as they could. But for the most part we would take care of them. The horses had a stable they slept in at night. but they all went outside to the big pasture in the day where they could graze and exercise.
The thing is, and again I may be mistaking conditioning for thought, but I always thought the horses were glad to see me. But again that might have been conditioning. I was the human that fed them. The ones that were going to be ridden I tried to saddle and bridle them before they left the stalls. Once they were in the pasture they sometimes like to play tag and run from me. but there were always a few that would come when I called. and once I got a few saddled the rest would show up and allow me to saddle them without playing tag. that might have been conditioning or herd instinct. the ones without saddles were going to be left behind. But back then I always got the impression that they enjoyed their daily trips.
Later on after college, I got to observe a horse logging operation. When I would stop the crew so I could talk to them the humans were always glad to shoot the **** with me. But the horses seemed impatient to get back to work. I always got the impression that their stomping and snorting could be interpreted as, "get out of the way buster, these logs aren't going to get down the hill by themselves." and the owners told me that the horses were always happy to get to work each morning.
Another consideration is domestication. Dogs have been domesticated for tens of thousands of years. and people have evolved with them. Fun fact. when a mother looks into a baby's eyes both mother and child have an increase of the "trust" or "love" hormone. We can detect similar increases in dogs and their owners. (1) I think my sister's pug/mix is much happier living with my sister than "in the wild".
Horses have also been domesticated for thousands of years. and I have first-hand experience of how much care a horse needs. I know there are wild horses and feral horses and maybe if horses could choose to be wild they would. but just like dogs, I think we can form a partnership with horses where both individuals can benefit.
Like many things I don't think you can apply any hard and fast rules. Observe, research and make your own decision.
When a dog looks into your eyes, it's bonding with you in the same way babies bond with their human moms
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