There are three secret monsters (Yeti, Mothman, and Flying Humanoid). To get the Yeti, you must first get the Waiter Skeleton Soul then go back to the very beginning of the game. You will see him walking in the background. Toss the curry and he will come up where you can attack him and gain his Soul. You will need the Rycuda Soul leveled up to at least two to get Mothman. 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. For the Flying Humanoid, you will need the Mandragora Soul. Enter the room in between the Zephyr Boss room and a save room, then use the Soul.