(Baby mouse or rat? Not that it makes a heck of a lot of difference!)
One year after a particularyly long seige of wet and rainy weather, the storm drains backed up into the sewer system and we rescued four baby rats that washed back up into our toilet bowl.
I remember walking by the bathroom and looking in to see something brown swimming like mad in the bowl.
I immediately shrieked with surprise and then slowly regained my rational mind. I went and got a Tupperware collander and a glass pot lid and dipped the little critter out and smacked the lid down on top so he couldn't escape into the house. I drained him, and took him outside and let him lose in the alley, came back into the house and found another baby rodent in the commode! This happened two more times.
We figured that the mother of these mice/rats had made her nest in one of the sewer pipes in a storm drain and the babies washed up into our toilet in a freaky turn of events.
Our dog, Phoebe, still will go into the bathroom and stare down into the toilet bowl wondering where the little naiads are. It cracks us up that she still remembers our bizarre little short-term guests!