The renk is a small, nocturnal, cave-dwelling slug. Its main use is in carrying a large amount of water: the creatures hold a surprising amount of water in their bodies, and can be consumed on long desert journeys as sort of a living waterskin. Each renk can hold 1/2 cup of water, so 32 raw renks can supply an entire days' worth of water in the desert.
They can live on the skin of other creatures, eating the sweat, oils, and dead skin of the creature.
A renk attached to someone while that person is in combat has a cumulative 10% chance per round of being struck by an attack that hits that person. Exposure to sunlight for more than 1d4 turns causes a renk to shrivel and die.
Occasionally, those who sell renk will cut the supply with leeches, so buyers should take care.