A while ago (maybe at around 600 days or so) I ran into this naturally skeleton who spawned with almost fully enchanted diamond armor. Of course I trapped it in a boat, but some seem to not believe me so I'm posting it here. Oh and the bow is enchanted too (although Im not sure what with) in case the screenshot isnt clear enough.

I cannot seem to add multiple images but I also found a full unenchanted diamond zombie recently on day 1725 (currently on 1802), but it sunk in a river and even though I got it out in time it still converted anyways even after leaving the water? So I actually have a full diamond drowned now. Those are the only two diamond mobs I have noticed/trapped so far.