People, cats, dogs, zoo animals, horses: the principle is the same.
Shelly Cone says, in “Nature’s Way” (Santa Maria Sun, Volume 13, Issue 27, September 13, 2012): “Through research, Jackson found that many problems and diseases common in domestic horses don’t plague wild horses—possibly because people have tried to change the way horses naturally live by putting shoes on them, keeping them alone in stables, and feeding them food they wouldn’t otherwise eat.”
Another idea in the article I like is: “Another aspect of horse behavior humans have tailored for convenience is tamping down the animals’ tendency to battle and bite each other. Jackson said humans teach horses—especially show horses—to settle down and behave, though that goes against their natural instincts.
Through such play, they establish dominance. They don’t just want to interact with each other in that way, they need to, Jackson said. As proof, the horses at Paddock Paradise bear bite scars and look a little wilder than stable horses.” (Copyright © Santa Maria Sun)