I had a panic attack when I changed my tranny fluid the first time...
That little thing popped out onto the floor and rolled under a tool box that hadn't been moved in like 10 years (the wheels were broken). So, we unloaded that sucker to get it light enough, jacked it up and sat it on a creeper, and rolled it out of the way. Quite a proud moment when we retrieved that sucker. Then, we spent the better part of half an hour holding it up under the damn tranny trying to figure out where it went and what it was. Finally, out of desperation, we made a call to the local tranny guy...
We described exactly what we had done, and then started to explain what the part looked like, at which point he started laughing...made us feel great! Seems he had pulled an AODE shortly after they were "new" and could not for the life of him figure out where that sucker went. Eventually, the Ford dealer told him to pitch it. He recommended we keep it and put it back in should I ever sell the car so that the next person to work on it also had to try and figure out what is was...
It is a plug. It is put in when the tranny is made, and when the car is assembled, they jam the dipstick tube into place and force that plug out into the pan. Where it sits until some unsuspecting soul finds it rolling around in the catch pan.
Either your tranny fluid has never been changed or they person who changed it put that thing back in there after. My guess is you are, in fact, the first to drop the pan. Which is a little scary...
Toss it. It is worthless.