That's not really for cars... it's used to block off pipes.
I've seen them under the hood once or twice but I dismissed those encounters as more DIY brilliance.
Careful when removing it, or reinserting the screw... the nut on the opposite side could get pushed out into the oil pan. The problem you're probably going to have is that if it is a plug such as those in the second picture the rubber has probably swelled and been cut into by the pan creating a very thick and tough grommet. I would place a very large fender washer on the screw, reinstall the screw enough so you're positive it's securely threaded into the nut on the back side and then try to get a small pry bar in there. You could also try using a hot air gun to heat the rubber and pan to make the job easier, but I wouldn't try a torch.
Can you post a picture?