Please help me identify what this is

[alvocado]

It could also be the oil pan leaking where it joins the transmission bell housing. There are areas on the two rear corners where RTV is required or the gasket may have failed. I would clean everything up then inspect after a couple of days as it's rather difficult to identify the source with that much oil residue.

 

[Geotrash]

Usually where the oil cooler lines mount to the side of the engine/pan above the oil filter (some have a block-off plate there if they don't have an external oil cooler) is the first spot to leak. Rear main leaks are less common than that one. If you pop that cover out and it's dry behind it you know it's probably not area main seal leak.