At the end of a long, tiring day, this is just what I wanted to find in my toilet. It was either a small / young rat or a big mouse. My initial reaction was to blink at it (as I was tired), do a double take, and then back away before shouting "Oh My Gawd!"
The lid was closed so it must have swam up the pipe to its untimely death. Part of me is amused, part of me had a morbid fascination for using it as a cadaver and part of me is hesitant to sit on my toilet again.