Help with bestiary

Hi, I'm missing only ONE monster, Mother Brain. The problem is I have spent a couple of hours in the Unknown Dungeon and this monster has not appeared yet, so I want to know if the monster is specifically in certain floors or something like that.

Thanx in advance.

I always find it on the top floor.

Many thanks. I looked in the final floor and in the second encounter the Mother Brain appeared, and in flock. :D

Thanx again.