I agree, but think it’s not a very important distinction. Certainly the owner could muck the stall and perform that job. But most owners of horses don’t like to do that and pay someone.
So I think a job is just a task that involves payment. It doesn’t change the argument. Someone has to do the task. If you give the horse owner UBI to free up more time, it’s unlikely to be spent shoveling horse manure.
In the case of horse shit, for sure it is better if someone cleans it, but that someone can also be the person that trains or rides the horse.
There isn't an intrinsic need for a job that performs only that task.