Hell, even during training there's a part called "breaking" the horse, which refers to breaking its will. Not sure if this is still done these days since the demand for horses is way down, but I can't see that being anything other than a form of torture for "problem" horses.

We came to the conclusion that keeping horses contained was a fantastic idea since they were an invaluable asset to our quality of life.

They were a mean of transportation, plowing our fields, gathering our wood, conquering and defending our lands.

The only part here is they lost their status as invaluable in the modernised world. Though their current status as "slaves" as you put it, isn't without benefit.

Few animals get the medical care a horse does, I'd even argue the average horse have better access to Healthcare than the average human. Gone are the days where a horse with a broken leg has a death sentence, so their quality of life have only improved in the last 100 years. And they will never have to worry about predators, not even humans, since they've for the most part been taken off our menu.

I'm not here to change your mind, I'm just saying, I think most animals would swap places with them in a heartbeat given the oppertunity.

If you didn't keep it in your field some idiot would probably just want to shoot it

Especially when you learn about how many were killed in WW1

Not quite the same thing, but in Hitchhiker's Guide to the Galaxy by Douglas Adams has the alien character Ford Prefect, who came to study Earth under the original impression that cars were the dominant life form. Hence his name.