I'm having a very hard time finding the last mind to 100% the game. This mind is the one left of the crab boss. What enemy should I kill in order to get it. I feel like I have already killed this enemy a bunch but never got the soul for it.

That's the grouper, the big red fish that first appears in the Hanging Pits.

I know which one it is now. Also, what is the drop rate for the mind?