I'm not sure if it's a bad idea or not, but Barats. Barats are the results of inter-species breeding between bats and rats. Either bats or rats were mutated by radiation to allow this to happen. They would originate from a cave in Idaho, and over the course of at least a century, the barats had become the dominant species of the cave and had grown to be almost as big as a human. I also feel like the barats could be sentient, but I don't have a good enough reason for it. It could just be that they eventually gained sentience naturally like humans, or maybe a large amount got shocked by something that entered the cave and the volts of electricity sort of acted like electric-shock therapy to them. I'm not the best at writing for Fallout so maybe someone else's strengths could work around my weaknesses? Or maybe they don't even need to be sentient, just really smart, like real rats.