It sometimes happens with feral horses/donkeys too. Mostly older individuals that have weakened tendons so get an uneven wear. They are the first ones to succumb to predators so are not often seen.
Generally, domestic horses that get overgrown hoofs have been shod before. To shoe a horse the 'nail' has to be trimmed, this removes the calluses that in nature grow on the bottom to walk on. Once a horse has been shod for a while, the ligaments tend to adjust to the way the animal stands. When shoes are removed permanently like on a retired horse, the hoofs should still be trimmed a special way for about one year to ensure that the wear is on the four natural contact points and build the muscles and tendons up to a more natural stand. A patch of hard ground in the paddock is also recommended
The teeth are designed for chewing tough grasses and keep growing throughout their life. Feeding non-natural feeds causes the teeth to overgrow as they don't get the proper wear. Once overgrown, it is hard to get them back to the proper natural shape so they may need continual dental attention.