Mulworms are domesticated animals in Dark Sun. They are large, poisonous caterpillars that nurture plant life around them, and who later turn into harmless butterflies that produce silk.
There are two main parts of this creature: the hazard of the poison, and the usefulness of the silk and the fertilizer they produce. Either way, they are not true combat threats. Instead, making them a hazard (with a useful side) makes the most sense.
The eight-inch long pale caterpillar sprays a gout of gruesome-looking green fluid towards your face.
Hazard: When the mulworm is injured or annoyed, it squirts poison at its foes. As they live in massive swarms, this can cause problems for those who discover them by accident. When the mulworm swarm is triggered, the triggering character must make a Dexterity save (DC 12) or be hit with the mulworm's poison. The poison deals 15 points of poison damage, and develop a rash that will last for the next 1d4 short rests. The rash imposes disadvantage on Charisma checks.
Countermeasures: An antitoxin is useful in leeching out the poisons.
Uses: Mulworms can have their poison harvested with a DC 12 Wisdom check. The poison is effective as a contact poison, but becomes even more deadly when injested or applied via injury, forcing the character to make a DC 14 Constitution check or die in 1d4 rounds. A successful check still causes the character to take 20 points of damage, and a penalty to future saves against mulworm poison.
In addition, mulworm larvae cause the trees they grow on to flourish, functioning much like fertilizer. The adult mulworm moth secrets a valuable silk, additionally. Mulworms can be purchased for 1 gp for a swarm.
World: Mulworms live in warm grasslands, and are uncommon there. They are farmed by some communities for their fertilization properties, and for their silk (and, less reputably, for their poison).
Story: Mulworms link assassination with the gardens of the rich, in which they harvest silk and berries nurtured by the mulworms. Mulworm poison can imply a rather large network of intrigue.
Mechanics: Mulworm poison is deadly when used as a weapon, but is otherwise merely a social inconvenience. It may affect the whole party, however, making it harmful if they are going into delicate negotiations.