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.