There are three secret monsters; Yeti, Mothman, and Flying Humanoid. To get Yetis soul, get the Waiter Skeleton Soul and then go back to the beginning of the game. You will see him walking in the background. Toss the curry and he will come up; attack him to get his soul. To get Mothmans soul, you need the Rycuda Sould at level 2. Go to The Pinnacle to the area where there are two final guards and a light in between them. Strike the generator next to the light with Rycuda and Mothman will appear. To get the Flying Humanoids soul, you will need the Mandragora Soul. Enter the room in between the Zephyr Boss room and a save room, then use the Soul.