It's fairly rare. It's also possible to miss, as there's no difference in appearance aside from the additional text stating a human/elf/drow/tanuki is being attacked. I've gotten into the "groove" of 1 and 2 key pressing and missed a few at times.
EDIT: Doing some more farming in the deeper region and after about 8-12 full cycles, came across it again. Didn't have the option for reward this time.
