I am not sure that there is actually a huge amount of pubic outrage'. It is the 'outrage' of a minority, a proportion of whom are anti-zoo anyway, fueled by an opportunistic and sensationalist media. Here, we have no evidence that there is any more issue with feeding horse, than any other meat. A small (very small) proportion of visitors will complain if they can see recognisable pieces of carcass being fed, presumably because they are used to having their own food sanitised and processed for them. Some zoos, of course, fuel this 'miss-education' and feed day-old chicks and other recognisable food items outside public opening hours.