In my experience, horses rarely actively enjoy being 'patted' or slapped(neither do I for that matter). Horses don't actively enjoy praise either. Therefore those things are not 'primary'(in behavioural term) or actual reinforcers(rewards) of themselves. Patting - & even a scratch, if the horse isn't into that - can in effect be primary *punishment* - that is, an UNdesirable thing. But when paired/associated repeatedly with actual reinforcement, whether positive(something desirable) or negative(removal of undesirable/pressure), those things can become 'secondary' reinforcers - that is, signals that the horse has 'done right' and will be/has been 'rewarded'.
I do think positive reinforcement(giving desirable consequence) is also really valuable, rather than only using negative reinforcement(removal of undesirable stimuli). I perceive the term 'reward' to mean +R, whereas Cherie & others use it to mean -R too.
Regardless what sort of 'rewards' you choose to use, as others have said, & Cherie's eg illustrates, it's vital to understand whether/when what you're doing to/for the horse is good, bad or indifferent, it's vital to understand the importance of timing of reinforcement/punishment and be aware, conscious of exactly what behaviours you're reinforcing/punishing.