I have a 2001 Mustang GT with an automatic transmission I remove the transmission pan to replace the fluid and filter when I did a white looking plug was inside the pan with an O ring around it I don't know where this plug goes I'm assuming its factory because the pan has never been removed can anyone help me identify this plug

 

Its the plug used at the factory during assembly. You can throw it out. They fill the tranny with oil with it out of the car. The plug keeps the fluid in. They then bolt the tranny up to the motor. Once its all together, and in the car, they stick the dipstick tube into the tranny which pushes the plug into the tranny pan. Its normal, and nothing to worry about.