I've punched my horses in the side several times. My wrist was in more danger than they were. It was quick, immediate, for a significant wrong behavior, and my horses accepted it totally. And it was done.
I did once kick a horse with my foot, but I limped for a week so that isn't a good idea. But no, the horse didn't become terrified of my feet!
But normally discipline via my body means either I'm blocking a mouthy horse with my elbow (blocking, letting the horse run into my elbow) or a quick punch in the base of the neck or shoulder. I'm not Jack Dempsey
(look him up, he was before my time but quite a character!). No horse I punch will ever suffer from it. And a punch is vastly better than looking for a lead rope, taking them to a round pen, and running them ragged - something I've seen too often. THAT is totally worthless as discipline. A quick, immediate smack on the shoulder works better. Horses understand it and accept it. At least the ones I've met all did...
Historical notes, just for fun:
"When I was a young fellow I was knocked down plenty. I wanted to stay down, but I couldn’t. I had to collect the two dollars for winning or go hungry. I had to get up. I was one of those hungry fighters. You could have hit me on the chin with a sledgehammer for five dollars. When you haven’t eaten for two days you’ll understand." - Jack Dempsey
"A champion is someone who gets up when he can't." - Jack Dempsey
"Though Dempsey rose to prominence with several brutal early-round knockouts, it was not until he lost his heavyweight crown in 1926 did he win over all fans. Back at the hotel, after being sliced and battered for 10 rounds by Gene Tunney, he was asked by actress Estelle Taylor, the second of his four wives, "What happened?"
"Honey," Dempsey said, "I forgot to duck."