I noticed this thread at the front page and could have sworn I gave an answer to it. It turns out, though, I only gave an answer to a similar question which can be found here.
In it I included some giraffids from the Miocene, which wouldn't exactly fit the bill for "realistic" (although the Pleistocene Sivatherium could hypothetically become a draft animal). Even though this post is over 3 years old and you may have already went through with your story, I'll see if I can answer anyway because what even are responsibilities?
In order to have an alternate "horse", it needs to share these similarities:
- It needs to be large and strong enough to ride and pull vehicles, or at least be feasibly bred to such a size later on (just like the real horse's history).
- It must be intelligent enough to train. Sorry, rhinos and ostriches...
- It must have the stamina to work for extended periods of time.
- It must have a comparable speed and have a quick acceleration to that speed, matching the horse's effectiveness in battle.
- A herding social structure is not vital to domestication, but will help immensely in becoming a substitute warhorse - the animal will instinctively charge with other mounted animals and be more likely to trust the rider's guidance. This is why donkeys are terrible in battle: they are much more solitary and self-reliant in the wild than horses.
So basing off of your criteria and mine, I will boil down my other answer into a few
specific animals that I think would be ideal (as well as adding some new information...so really, not much of a 'boiling down').
- Zebras: Okay, okay, I'm technically cheating here, but couldn't resist being cheeky. You said no horses, but never said anything about equines in general! Unlike the parroted words from Guns, Germs and Steel that you may often hear, this is actually more doable than you think. I'm actually working on a very long Quora answer about zebras which pooh-pooh's Diamond's claims on the subject; I'll be sure to link it when I'm done.
To put it short, though, zebra "domestication" was much more successful than people remember (albeit low-intensity), went on for several decades before being replaced by more efficient horse medicine and modern vehicles, most zebra species share the exact same social structure as a horse, zebra behavior towards humans is little different from the tarpan, and among all the species the most readily tamed animal was the now-extinct quagga, which would then have a temperament not unlike a domestic horse which is beyond impressive for a wild animal.
So you could do it if you wanted. Of these, the ideal "horse" replacements would likely be one of the plains zebras; either the Burchell's Zebra or the quagga. The Grévy's zebra handles more like a donkey than the plains zebras. They all might be too close to horses for your liking, though!
A final note, horses and zebras are both capable of some light browsing and so could probably make it on the periphery of your jungle habitat, but may have nutritional deficiencies if they stay too long.
- Bovins: My gut instinct go with some long-legged variant of domestic cattle, since this is of course the animal we know the most about. OTL, we have longhorns, which have a more cursorial build than other cattle and are popular to ride. Yaks could work, were it not for the fact that they aren't adapted well to your world's climate. And water buffalo are great alone in your jungles, but their wide feet would make for terrible long-distance runners. Cattle are well-adjusted to your world. They can handle a wider variety of feed than horses, and, being ruminants, can digest their food more efficiently which will be very helpful in the jungle.
Alternatively, and if you wanted to spice things up, you could go with the banteng. They have the affinity for dense, wet tropical forests like the water buffalo, but have a build more like regular cattle which makes developing running breeds easier. They're only somewhat domesticated IOTL and so we have very little information to go on, which makes this risky for a 'realistic' mount because of the assumptions we have to make. But if you assume they're like cattle, which I don't think is too 'out there', then they would be quite ideal for your world.
The biggest quip about bovins (but not other bovines) is that their backs tend to be very flat, leading to a rather more uncomfortable ride than a horse. Any long-term riders will be very bowlegged...but this could potentially be solved in ATL breeds.
Addendum: Any kind of head-butting mount will have interesting implications in war, especially when faced with another head-butting animal...
- Camelids: The dromedary camel, or a surviving Hemiauchenia. Camels can approach some horses in speed, but the fastest camel is still slower than the fastest horse, can't keep up their top speed for long and aren't as agile or quick to accelerate. The size of a camel can make cutting down people difficult, but doable with a lance or long sword. Camel archery is also feasible.
Dromedaries are much more adaptable to hot, humid weather than Bactrians, but will still require ATL breeds to survive in actual jungle. Hemiauchenia has been found from Nebraska to Florida, so not only may it prove the most adaptable, but may also be the faster and more agile pick. Beware the giant llama.
- Antelope: The giant eland would be the best pick for riding-type domestic antelope. I talk very favorably about eland domestication in this post. In 1924, Arthur Blayney Percival, member of the African Game Department, speaks highly of the eland's speed and ability, at least for the cows (he may be talking about the common eland), and says they are "easily trained to harness". Harry Edgell, who lived in German East Africa and had been performing hybridization experiments on tame zebras, said that two elands in double harness "trotted thirteen miles without a break". Hugh Cholmondeley, 3rd Baron Delamere apparently had similar success breaking in elands, but I can't find a more direct source.
Though the common eland is more...common in a farming context, both species are farmed and ranched. They're a safe bet. They can eat a wide variety of plant material and are hardy in dry areas, but might have some problems in densely forested areas.
- Deer: Any large member of Cervus canadensis, though I'd go with the Eastern elk for their size and ubiquity. Reindeer would be the obvious answer if it weren't for the fact that they'd all die in your world. Compared to the eland, there isn't as much solid information on taming apart from anecdotes and rumors. However, it's safe to assume that they're much the same as the smaller Rocky Mountain elk, which are easily tamed and are farmed today for meat and antlers. If you've ever had an elk burger at Fuddruckers, it was from a semi-domestic elk bred in captivity. There also exist at least two ambiguous photos of a man riding an elk, which may be Roosevelt elk from their size.
Elk are highly adaptable generalists and can survive on the open grasslands, dense forests, hot, dry, humid, cold, whatever. They will eat almost anything and digest it more efficiently than cattle. If you have a ridable breed, then I think these are the ones best equipped to handle all of your world's biomes.
Unlike many bovines which headbutt and slowly back away for another impact, nearly all deer, elk included, fight by constantly pushing forward and lock antlers (exemplified well in this video). Unless you're using cow elk, that would make for an interesting battle dynamic.
That's pretty much everything as far as "realistic" goes. To get more diverse animals we'd need to go back in time further than realism would allow, starting with pre-Interchange South American megafauna. Though who doesn't want to ride a terrorbird...assuming you can keep your balance and your limbs?
It's hard to rank these animals from best to least suited because they're all so close together and have their benefits and drawbacks, However, we may not have to split the difference: you could have separate animals for open and forested terrain. Zebras, elands and dromedaries seem the best equipped for just the plains and light forest, cattle and potentially Hemiauchenia are excellent plains mounts that can handle incursions into the jungle, and banteng and elk are the best equipped for life in dense forests, though elk can thrive in nearly all of these biomes.
For that reason, I'm really tempted to call elk the "winner" here, despite the lack of domestication data and a close match with more specialized animals.