Any ideas on where this leak is coming from? I believe it’s an oil pan leak? But I’m not quite sure what that black plug looking thing is... any ideas?
Here's an idea: give us the year/model of the Mustang, engine and trans type...and take a picture further away that captures the area you are looking at...
Well....I can tell you that the "black plug" isn't designed to seal anything. It's a inspection cover for the transmission bell housing.
Typically if there's oil coming from the transmission inspection plug the leak source could be:
rear main seal
oil pan
transmission oil pump front seal
What is the condition of the transmission fluid? Bright red or brown and burnt?
Bottom line. If you really want to know, the first thing is to determine if the fluid is motor oil or transmission fluid.
Next. clean the area and make note of where new leaks come from.
Hint. an oil pan gasket rarely leaks from just the rear of the motor. If it's the oil pan the leak will be in other areas around the bottom of the block.