Probably Benzilla. He showed up at just the right time, and I finally got to use the anime book.
Ooooh, "at just the right time", what do you mean?
I think it was towards the end. I thought I had seen all of the monsters, and then he showed up.
Man, that's even cooler! Benzilla is a rare encounter overall, but I think he's much more rare in that last dungeon, because every single enemy in the game can spawn there.