Slappy appears as two different bosses. One is EITC, and the other one is a Ghost.
A level 99 boss in the cave of lost souls on the walkway near Foulberto. He has 2 Rage Ghost guards, and 300,000 health. After every attack, he only gains health. To defeat him, you must use silver freeze or cursed ice then have an army of blunderbusses.