In Red Dead Redemption 2 (opens in new tab), only humans have the privilege of riding a horse, but thankfully there's now a mod that fixes this inequality by letting every living creature in the game ride them. Everything from a bear to other horses can now enjoy the experience of being ferried from A to B while resting their tired legs. It's absolutely terrifying.
Clearly neither animal is particularly happy about the state of affairs, perhaps realising, innately, that this is completely unnatural. They're not willing participants, either. Pir2's Horse TP mod (opens in new tab) lets you teleport animals onto a horse you've assigned at the press of a button, at which point they miraculously appear on your horse's back, probably pretty surprised.
Larger animals like bears and boars clip through the horse a bit, making the pair look like some sort of Cronenberg nightmare. Smaller animals have a better time, like this bird that actually looks pretty comfortable.
You can only teleport one animal onto your horse at a time, with one exception. If you teleport another horse onto your horse, and someone is riding it, both will be teleported. Humans get predictably pissed off when you use your black magic on them, however, so expect them to start shooting.
At the moment this only works with horses, but who knows? Maybe Red Dead Redemption 2's frontier will eventually be full of bears riding slightly larger bears. But hopefully not.