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. Also, most bugs living within the Human Empire boundaries are allied with them, because otherwise it would be similar to nuclear war. Humans use poisons, bugs spread diseases, no one wins. The bugs outside of the human empire are with the UAR. 
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.