>Native? But where did they (bison) originally come from? Even if they were here for tens of thousands of years -- at some point, they weren't, right?

Correct. The earth isn't native either it came from a collection of space dust drawn together by gravity. The sun isn't native, or our galaxy. I guess the universe could be considered native unless you believe in multiverse hypothesis, in which case the big bang could have come from an interaction with another universe. In other words don't kill anything and moo!