Think tank time
Suggest something that could be used in a swampy environment as transport and could survive naturally within that environment. Attention to detail is a must, otherwise Souzou's ocd will immediately dismiss it.
I'll start this off.
Giant Swamp Chicken:
Standing @ 5-6m tall, the Swamp Chicken(Moorhen) can traverse through water up to 2-3m in depth with relative ease and once on a more stable ground can move efficiently for great distances. Being bipedal, it requires less calories for it's locomotion. It is, however, slower than a horse when mounted to allow a more comfortable ride.
It's feathers allow it to acclimate to the varying temperatures from morning to night. It can shed water, hold air to keep warmer or ruffle up to create an air flow. Also, the Swamp Chicken can open it's beak when it needs to cool off in the heat of the day while running.
The Swamp Chicken can eat anything. It's got an iron gut built in and is omnivorous. It also has strong predatory instincts, making it an ideal replacement for the War Trison on the battlefield.
Edit:
It has been mentioned, and I forgot to mention here. But there are in fact 4 unique mounts in total available through EP and Titles(5 if you include purebred horse). Not everything has to be a war mount. Pack Mule(move stuff), Horse(move quick), Ursaphant(move more stuff) & War Trison(move into battle) replacements each might need to be considered.
Also, good info for cart/wagon replacements seem to go hand-in-hand with this topic. Already, many have posted good ideas in that regard.
Look at those Talons!