That's just it though. The implication has been that there aren't any others left. What wasn't wiped out by the god-kings was wiped out by karma demons, fiends, or warped by the subconscious collective within the Holy Barrier.

Queerrats are quite a bit more intelligent than any of the other warped species (monoshiros, exploding flycatchers, blowdogs, etc.) that it might be more likely they didn't start as an animal, but are de-evolved humans.

And the most savage of the queerrat tribes are from across the sea. The closer to the civilization, the more advanced they are, the more humanlike. It could go one way, or the other. Maybe they did start as mice or rats and become more and more evolved the closer they get. At this point I'm no longer sure which.