Say somewhere in the deep wilderness where no human has ever reached, perhaps in some dark uncontacted continent, a large group of a certain pokemon species have achieved a level of population and organization comparable to that of Mesopotamia at the time of the invention of writing, around 5,000 years ago in our history. Assume they've never heard of humans and our culture has not impacted theirs in any way.

Which species do you see as likely to achieve this? How would they treat other pokemon species, or humans if they ever came across any?