For my Dark Tidings setting, I decided to challenge myself by not using about 99.9% of the monster manuals/bestiaries.
One of the monsters I came up with is called a bloodcloak. Take a torso and head, open up the ribcage and remove all the organs. Remove all skin from the neck down. Using demonic magics, give it the ability to fly through the air like a ray does in water. Oh, and the worst part: not only is the poor sap still alive, they were awake during the process that turned them into what they now are.
Almost completely useless in a fight, except for ambushing a lone target, demon cults use them primarily as scouts and lookouts. If they do find a lone target, they will swoop down on them and strangle them with their spines-turned-tails.
It is rare to find someone outside of a demon cult that doesn't view destroying a bloodcloak as an act of mercy.