Do horses miss their owners?
It depends. A horse is not a dog, usually horses live where there are other horses around which means that the horse can have a vibrant social life without a human around.
IF a horse lives without other horses around then that horse could well miss their owner, just for the social life aspect. This type of social life is usually nowhere as good as living with other horses though many horses adjust especially if they have a good and sensitive owner who spends time with the horse.
If the horse's owner feeds the horse regularly the owner WILL be missed if the food does not appear. Once food appears it is usually back to a normal outlook on life.
We love our horses. Our horses do not "love" us, at best the horse considers its human a friend, a non-violent friend who works with the horse instead of against it. If the owner is harsh, rides badly, punishes the horse constantly, and won't work with the horse that owner will NOT be missed if that owner does not feed the horse at the usual meal times.
And feeding a horse unearned treats does not grow a friendship, it causes the horse to learn to disrespect its human because the owner is a dispenser of candy and not much good for anything else. If the horse wants a treat and the owner does not provide a treat, this is often when the horse biting people starts. Do NOT try to buy your horse's love with treats, you will ruin your horse as it becomes a dangerous biter.
We are reliefs from boredom. We are the scratchers of all those itchy places the horse cannot reach. Most horses learn to put up with their owner's humongous imperfections as horse people, especially if their owner feeds and grooms them.