What is this???

Hi
I am changing Automatic Transmission Fluid in my girlfriend 2001 3.8l Mustang. When I drain oil and removed oil pan I have found this plastic thing youst lying. I have no idea where it comes from. Any ideas? This is like 5cm long piece of plastic with rubber ring.
Many thanks for help!

they put those in on the assembly line so nothing gets into the tranny befor they install the dip stick tube, the person on the line was to lazy to pull it out so they just popped it inside while installing the dipstick

Its called a lollipop. its used to help guide in the manufacturing process when they have to install the tranny fluid dipstick. it is very common that it was left in there as they usually do it wont hurt anything just throw it away