A cuuyn is a strange, slow-moving creature that can only survive in areas where the air contains a certain, top-secret compound. A cuuyn's lifespan is directly correlated with the density of the compound it breathes: the denser, the longer the cuuyn will live, with no documented upper limit. As a cuuyn ages, its bodily cycles speed up exponentially, significantly increasing its metabolism, gestation period, and immune systems.

It is widely believed that cuuyns were artificially created for government animal testing, but no one has ever found proof.