I second the Coral Adder. I have only seen a few, and none were rares.
I don't have a rare minfernal yet either. But I think I will just stone the uncommon I have and move on with life. I have many pets to lvl to be camping for weeks. But I'm pretty stuborn, so I might try a few days first.
Still have not found a rare adder. Amazes me at how few spawn. It is my last rare wild that I need, so my next beast stone is probably going to finish it off.
I am hoping that the drop rate on stones in the new 5.2 battles will be significant (seem to remember reading that), making the upgrade of my remaining uncommons an easier row.