Neochoerus zefrankii - the giant capybara
Wild
I always wondered which all the carnivores feasted on in the swamp region until I met a group of capybara like creatures. However those giant rodents are far bigger than their counterparts and roam in large groups between land and water. Before I could watch them closely those shy creatures vanished in deeper waters.
Taming
Like it's counterpart Neochoerus has no way on its own to produce vitamin c, so it has to find rich sources for it's diet. It's shy nature prevents a direct taming method and tribes came up with the idea to plant citrus fields to lure it in pens until it can be approached.
Domesticated
Tribes usually have a different name for neochoerus: Crappybara. It's unique behaviour to digest it's own poo results in a secondary poo which can be used as an enhanced fertiliser.
Furthermore, it's anal glands secrete an oily substance which is an effective repellant working against the smaller carnivores of the swamp. I even heard of tribes rubbing each other with it to safely dive into shark-infested waters.
Explanation:
Based on Neochoerus pinckneyi and recent counterparts, the Crappybara is no endgame creature (or intended to be) but an addition to the middle sized creatures located somewhere between the phiomia and and a thyla. It's extinct predecessor managed to extend it's habitat from south america toward southern north-america and was approximately 40% bigger than modern capybaras.
It lives near any bodies of freshwater or swamps and will become untameable if touched or a human comes near it.
It serves as an alternative to the dung beetle with an extra clou: once feeded by any creatutres poo it produces special poo, which raises instantly crops from seed to full grown plant.
It can also be used as ingredient for a shark repellent, so no creature til the size of a megalodon will attack you and your mount in water.
It' a semi-aquatic mount to crosstravel the swamps and rivers as its special odour and size prevents it (and the rider) to be attacked by the smaller wild carnivores like titanoboa, dilophosaur and even kaprosuchus.
To tame neochoerus you need to collect spent secial poo in the wild or by looting dead capybaras. Then you build citrus crop plots (fast grown with the fertiliser) in some distance to them and wait till they come raiding your fields. Depending the creature's level and the plot count you need to refill the plots until its tamed.
https://en.wikipedia.org/wiki/Neochoerus_pinckneyi https://en.wikipedia.org/wiki/Great_American_Interchange