Mongippo

Mongippo is a future descendants of omnivorous mongooses that evolved into gigantic hippo-sized herbivorous amphibious mammals that fill a similar niche to African hippos. They mainly feed on grass and underwater plants and, despite competition from hippostriches, mongippos flourish really well.

Evolution
After the Late Holocene ended, humans are now gone, so niches were left open in North America for many animal species. Mongooses that had escaped from zoos faced competition from mustlids, so they became more herbivorous, grew bigger, and became semi-aquatic, reducing competition from the mustlids. Over time, they evolved into gigantic herbivorous semi-aquatic aggressive mammals known as mongippos.