The Beast is the hidden antagonist of the Billy & Mandy episode "Sickly Sweet". He created the Mask of the Beast in his own liking.
The Beast wanted the rest of the Underworld to fear him and made a mask in his own liking. The Grim Reaper took the mask and kept it inside his robe so that no mere mortal can touch his mask.
Like the wearer of the mask, he has donkey ears, horseshoes, fire breath, a tail, claws, purple skin and green slanted eyes.