As creatures of primitive instinct, zombies often engage in behaviors crudely mimicking the functions of their former lives; for example, zombies occasionally attempt to "mate." Their random distribution of living and nonliving matter often renders the very act impossible, and their heavily mutated genes cannot possibly produce a human or even literal "zombie child," but when relatively whole where it matters, the union of two shambling corpses can sometimes result in a unique mutant organism colloquially known as the Zombryoid.

   These unnatural embryonic organisms, also called "corpse larva," grow to several inches in length within mere hours of conception, and are already able to survive outside their mother's rancid body. Many will remain permanently in this primitive state, seeking dark, damp places and sponging nutrients from decaying matter. Others will evolve gradually into the creatures called "Festerlings," and a rare few will harden into a sedentary pupal form, the Sarchrysalis, to metamorphose into such dread beasts as the Progenoid, Zyghast and Grimgug.

   Another of these final forms, the Psychophagus, never actually leaves its pupal shell. Consisting of almost 95% brain tissue, it is the only zombie-derived monster capable of generating its own psychic energy, which it uses to telepathically lure and mesmerize living creatures. Dazed and oblivious, the prey is penetrated and digested from within by the monster's subterranean "root" system. The sole visual difference between a Psychophagus and Sarchrysalis are the latter's perpetually closed eyes.