My parent's neighbor flushed her to toilet today and up came a RAT! She shut the lid and ran over to get my dad to help her. When he opened the lid he realized it was still ALIVE. Rats can hold their breath up to 15 minutes allowing them to swim up pipes. This problem is more common than you think- having blogs dedicated to it! Apparently, some major cities are experience rat-toilet epidemics.
Here's what to do:
Immediately shut the lid. Squirt liquid dish soap in the side. This decreases the oils in the fur so it cannot stay afloat in the water. Then, after it starts to sink, you can flush it.
However... without seeing this tip before the rat problem, my dad just used a plunger to drown it. SO GROSS! But, hey, what are good neighbors for :)