There are a lot of animals that are deadly to humans. A pissed off chimp can totally ruin someone’s day (they tend to remove people’s faces and fingers). Lions, tigers, and other big cats can take down a human. Delicious is correct that hippos are deadly. Elephants have killed people.
There is a big difference between an animal that MIGHT kill a human if circumstances are right and an animal that WILL kill a human on command.
Chimps tend to avoid humans and avoid confrontation in the wild unless they feel threatened, so merely pointing your chimp army at the enemy isn’t likely to be productive. Wolves very rarely attack humans unless provoked, and often break off their attacks if the human fights back. Unless they are starving, the wolf pack would usually prefer to just go off and find easier prey.
Chimps are pretty smart, so I think they could be trained.
Wolves aren’t as easily trained as their dog relatives, but wolves can be trained, so I would put them on the list.
The OP already mentions dogs. They have a proven history of being used in warfare, so they are definitely on the list.
Elephants were used in wars in years past, so they definitely go on the list.
Hippos are notoriously unpredictable. Even if you use the cattle prod method that Delicious proposed, they would probably be as likely to turn and attack the cattle prod handler as anything else. Hippos also need constant water and mud or their skin dries out, which makes them unsuited for battle away from their watering hole.
Rhinos would make a formidable war animal, but they were never used in battles in the past since they basically can’t be trained. That said, there are some ancient wood carvings that depict rhinos in battle or in armor. So put this one on the list as a maybe, with more research needed.
Horses have been used in war, even in surprisingly recent times. Horses are naturally skittish, but you could probably train a horse to attack a human via bites and kicks. Horses don’t exactly have a hunting instinct, though. There’s a reason they were used as mounts and as pack animals instead of just turning them loose on the enemy.
Big cats aren’t typically used in battle, but Ramses II supposedly had a lion that he took into battle with him.
Bears can be used in battle. Private Wojtek (a bear) even carried artillery shells during WWII. To this day, the Polish 22nd Artillery Company’s emblem is that of a bear carrying an artillery shell.
The ancient Romans would sometimes catapult bee hives over their enemy’s defenses. So I guess we can add bees to the list.
Stampeding cattle have been used in battle, with mixed results. They can be effective, but they can also be easily driven off in the wrong direction.
Scorpion bombs (literally clay pots filled with scorpions catapulted at the enemy) were used in ancient times, in areas where scorpions are prevalent. From what I have read they were a rather effective weapon.
If you get a little loose with the definition of “trained”, there was also the attempt to use swarms of bats to firebomb Tokyo in WWII. The bats weren’t really trained. They were just relying on the bat’s natural instincts to seek shelter, and the mostly wood and paper construction of Japanese homes at the time. Still, weaponized bats are definitely a thing that a lot of research went into at least once.
But bee hives produce honey, a valuable product. Even more so in ancient Rome.
Wouldn’t it work just as well to use hornet nests? They are a very nasty pest, best gotten rid of, and produce nothing worthwhile.
also, dolphins might be included here.
Didn’t the military try to train dolphins to swim into harbors under enemy ships with suicide bombs on their backs?
I had though about dolphins and killer whales… we keep them in aquariums and train them to do amusing tricks without killing us (usually). Probably not feasible but couldn’t you train a pod of Orcas to cruise along swimming beaches and mow down the swimmers… or send them into enemy harbors to take out their military divers protecting their own boats from underwater threats?
Hyena handlers in Nigeria have figured out how to control some pretty nasty predators and have them perform… I wonder if they could train them to attack as well?
Australian currawongs, butcher-birds and magpies, which resemble but I think are not closely related to Old World crows and magpies, regularly dive-bomb schoolkids, especially during nesting season. They also form strong friendships with people who are nice to them [nice=food]. And they are as smart as.
Yes, Orcas is the correct answer. They are the apex predator of the sea. I have no doubt they’d be top predator on land and sky if they could travel in those environments. Well, with a little high-tech help, they could. That they are super-smart and trainable is a bonus.
Suspend a pod of trained assassin orcas from large drones. Put one of those retina-tracking fighter pilot helmets on each one of them (the plane flies where they focus their eyes). Point your nemesis out to your hit “pod” and yell, “sic em!"
If a swarm of killer whales started dive bombing me from the sky with mouths agape, I’d call it a day and commit hari-kari.