I disagree - I think most people would say a dog or a horse are quite intelligent and have "souls". But dogs and horses don't have the conceptual ideas of freedom and economy, and they enjoy doing the work they're given (in part because we spent hundreds to thousands of years breeding them to do that).
So as long as we're kind and treat them well, nobody has a problem with their slavery. The idea being that exploitation is linked to suffering, but simply having an asymmetrical relationship is not inherently cruelty.
Not to be crass, but we can also bring up other concepts like BDSM. Violence and domestic abuse are objectively bad. But in the context of a safe relationship where all parties are informed, consenting, and enjoy the abuse they're receiving... Now violence and domestic abuse are pretty clearly morally neutral.
Horses know they are slaves and don't like it. But they aren't very smart, and like not very smart people, they take petty revenge when the opportunity arises. For example:
1. if you walk behind a work horse, he's likely to kick you
2. if you ride a horse, he'll try to run under a branch to knock you off
3. a horse will take a deep breath and hold it while you cinch a strap over its chest. Then the horse will exhale, and the cowboy will have to cinch it up again. What the savvy cowboy does is pull on the strap until the horse gives up and takes a breath, then cinch it hard
4. to work with horses, you've got to let them know you are in charge and won't take any nonsense from them