Yeah that's why I'm picturing 19th century having the Columbian drug hippo problem on steroids. Can anyone say hippo fishing with dynamite...Hippos make terrible livestock... more people in Africa are killed every year by hippos than by crocodiles
The elephant thing is intriguing, although the phrase "large slow target" does come to mind.... I could see some uses though, elephants are highly intelligent and useful if handled well, properly... almost like having a very large, strong paid laborer. In one place in Asia laboring elephants even know from habit what their days off are
The US Cavalry did attempt a Camel Corps in the southwest once, not very successfully... even hired an Ottoman Turk (I think of Greek ancestry) named Hajj Ali (or "Hi Jolly" to the locals ) to assist in the training and handling...
I could see the elephants being used to pull heavy artillery around the front or to haul wagon trains.
There's also that one guy from South Africa who trained an ape to man the switches of a railroad station, paying it with beer.