[quote=jeray2000]
I excluded other large animals like rhinos because they don't live in the Americas, or they were among the many trying to create feudal holds in South America. [/quote]

Understandable, but I've reached a conclusion with Important, I believe. 

[quote=jeray2000] But which animals are the bad guys again, and where would their territory be? I was thinking maybe the plot this revolves around could be a kingdom rising in South America where several evil animals united to destroy the humans and the UAR.
[/quote]

I'd think species that have a reputation for being wicked, like snakes and other reptilians, could be the antagonists that want to take the deserts of North America for some reason or these reptilians see themselves as the 'most powerful' species and wish to destroy/enslave the UAR.