Some monsters from earlier editions of D&D do not have monster stat bocks, but instead have been classified as Dungeon Hazards, which essentially function as traps. Such creatures as Green Slime, Yellow Mold and Brown Mold, are described on page 105 of the 5th Edition Dungeon Master's Guide.
One classic nasty little creature(s), the Rot Grub, would make a perfect Dungeon Hazard to give those greedy corpse-robbing player characters something to think twice about the next time they want to go rummaging through that dung or garbage pile.
Rot grubs reside in rotting substances, most commonly the rotted flesh of corpses, but they can also reside in offal, dung and rotting vegetation. They resemble maggots of a size of about an inch long. They viciously burrow into any living flesh which they come into contact with.
Anyone inspecting an object infested with rot grubs can make a DC 14 Wisdom (Perception) check to notice any signs of infestation. Anyone touching an object infested with rot grubs must make a DC 12 Dexterity Saving Throw or be exposed to the rot grubs which will emerge and begin to burrow into the person's limb or other extremity that made contact. If the creature was wearing gloves or is fully armored or clothed in a way that minimizes the exposure of bare flesh, they may make the saving throw with advantage. The rot grubs do 3 (1d6) piercing damage on the first round of contact. If fire is immediately applied to the rot grubs, they will be killed immediately, however the creature will also take 3 (1d6) fire damage if from say, a torch. A fire-based spell will do it's damage if used to kill the rot grubs.
If no flame is used to kill the rot grubs on the first round they make contact, they then burrow into the flesh of the creature on the start of the next round. After this point, fire cannot be used to kill them, as they are too deep within the flesh to be harmed. The rot grubs then burrow towards the creature's heart. A Paladin's Lay on Hands class ability that is used to cure disease will kill the rot grubs, as will a lesser restoration spell. Another alternative is decapitating the limb. This is effectively a critical hit using a slashing weapon that does maximum damage.
If the rot grubs reach the creature's heart, it immediately drops to 0 hit points. Each round the rot grubs do 3 (1d6) necrotic damage and reduces the creature's hit point maximum by this amount.