Are they listed on "region-pops list" or "region-pops list-all"? If all they're wandering in from 'outside' and aren't thus 'local'. "region-pops incr-all DINGO (number)" would be what you need.
The exact set that you should be able to modify is printed by list-all, i.e. in other words if it isn't there, it won't be recognized. List without all should be printing only animals that you've already actually had arrive on your map, or something like that.
They can't have wandered in, since their population is 0. 3 of the worlds I've tested have no cold regions, and thus no polar bears or others even with "list-all," but the cave creatures are still listed as population 0. I'm trying to solve the bug where cave creatures vanish after a while of playing adventure mode, because I want to encounter them in adventure mode. I just used Dingoes as an example because I had seen them. "region pops-list" in adventure mode returns nothing.
I did not know that you had to specify the creature after incr-all, I just thought that it incremented ALL populations, so that explains why my attempts weren't working. I'll try it later, see if it works.