We don't do anything. We'll have to take care of them, which is where things like a basic income help. It won't happen all at once instantly, and it's not like our only option is to grind them into soylent green. Horses may get turned into food, but that's because they're horses and not people. We eat animals, so it's not very compelling to say "oh no, we ate all the horses or turned them into glue". Who cares. The point is there are horses now and there will probably continue to be horses. Many of them with a life of mostly leisure and good care instead of being worked to death. Sounds good to me.