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I was looking for an oil leak on the passenger side (more later) and I found this between the passenger side frame rail and the oil pan. I have no obvious clues as to where it came from. The side may be torn, as the are irregular but smooth....
Are there any noticeable issues with your Engine Mounts?
Check your Dogbone mount near the Passenger side strut tower, if that's not it, check the mount below it, that's all I could think of being as big and rubbery in that area. If that's not the mounts, inspect the general area where you found the piece, if all that is good, it may have just got trapped in there during normal driving or during the maintenance.
The dog bone and motor mounts appear fine and nothing is banging around in the suspension.
Possibly road debris?
Other than some rubber mount in the engine bay, I can't think of anything else. My 2000 ES300 had a different design of the rear main mount from yours, and this piece looked close to what you can get from there, but since you have a different design, there is no way it is a part of that mount. The good news is that if a piece like this is missing from a key component, trust me, you will notice it, so I wouldn't worry about too much, just be sure to keep an eye on it.
Yes, I had it pretty far apart to replace a leaking rear cam seal . The dog bone is fine, the motor mount does not have a comparable part and appears fine. Nothing bumping around in the suspension. So, I am going to write it off to road debris.
Last fall I looked at the rear valve cover gasket and spark plug seals. No signs of leaking so I stopped at doing the plugs and coils back there. Sadly, the cam seal caught me off guard.
Help identifying this part....please
. The dog bone is fine, the motor mount does not have a comparable part and appears fine. Nothing bumping around in the suspension. So, I am going to write it off to road debris. 
