This is an interesting but useful topic. I think everyone who owns a car has had an experience of a bug/moth/bee/mouse and rat inside the car! While its not an alarming situation mostly, it becomes dangerous if someone's driving the car! You have to check your own reactions and reflexes! Panic can be very dangerous if you are driving.
If you find a lizard, rat or bug in the car like spider etc, try NOT to use any killer spray! The trapped insect or animal will try to hide and later on die, leaving you with a foul smell. You might have to get your seats removed, or in worst situations, even the dash! BEST option, workable in most of the situation is already given by Storm; "Park your car in direct sunlight, with all windows closed less one (front driver or second seat)". In a couple of hours, you'll find that the uninvited friend of yours has managed to find an escape route and got out. In case of a rat or frog, you might have to leave a door slightly opened as these reptiles wont jump out of the opened window.
In some other serious situations where this trick fails, heavily spray in your car some good anti insect/raptile spray, and again keep all doors and windows closed except one. Fresh air will lead the reptile out.
I once had a lizard in my car, which managed to stay behind dash for four days! I never mind its presence as it was mostly not visible, but once my kids saw it, I had to use trick-1 mentioned above! Not that my kids were afraid of that lizard, but they were making strange and extremely dangerous (for my car) plans to kill it!!!! I was sure if any of those plan was implemented by them, my dash was a history! One of the plan being discussed was to use a burning candle and roast the butt of the lizard behind the dash!!!!! What initiative!!!!?!
Good info sharing! Hope someone will surely benefit from this discussion.
Have a safe ride!