I'm basing my statement about squirrels ~entirely off of the fact that IRL we have domesticated dogs and googled statblocks says the 5e squirrels have Int=2 and Mastiffs have Int=3. On the other hand, I have seen videos about squirrels participating in obstacle courses to get food, so who knows really...
Ah, but BG1 had this wand of polymorphing, which could turn people into squirrels. Perhaps BG3's rodents are the descendants of such unfortunate characters.
