Renk are small slugs from Dark Sun that produce hold substantial quantities of water, making them useful on long desert trips.
Rather than featuring as monsters, as they are not a threat, they are perhaps better regarded as items that the party can employ, more equipment than beast.
This fat slug glistens with moisture.
Equipment (1 sp): The renk is useful for providing water for long desert journeys, as they are lighter than carrying fluid with you. A day's supply of food and water can be held in about 30 renk, which is about half the encumbrance of a normal waterskin. The renk must be eaten live to gain the moisture from them.
World: Renk are used by those who travel long distances. They are subterranean creatures in the wild, living under rocks and in grain basements and wells.
Story: A colony of renk can be a lifesaver for a party low on supplies in the middle of the desert. It can also serve as a trap for such a party, luring them into an encounter with whatever holds the renks.
Mechanics: Renks are only useful when you are tracking encumbrance and rationing food and water, helping a party travel light while supplying their needs.