There is only one type of Gestorumque, and every kaiju that appears in the Nemesis Saga is the product of deliberate genetic tinkering by the Aeros.
Project 741 reveals that the black armor of Nemesis and its ilk is a symbiotic fungus and thus a separate organism. While this is barely specified further, it does imply that Nemesis's "Divine Retribution" mode is in fact its true form. Since Nemesis was created through torture and experimentation by the Aeros, it is entirely possible that Nemesis Prime came to be in its displayed form via artificially induced hybridization/chimerisation with an alien fungal parasite. Furthermore, Project Hyperion all but states Lovecraft to be some hybrid/chimera of a Gestorumque and an Aeros, while the idiosyncracies of Giger's appearance seem highly evocative of a Ferox. Since the implications would make all "base" kaiju variations of the same entity, the rest being either Prime's offspring or a parasitic imitation of Nemesis, it is safe to assume that Gestorumque is simply the term for either a single lifeform or a homogenous race thereof.
- This possibility would also open the door to a convenient instance of continuity welding with the Antarktos Saga: As mentioned in the YMMV section, the Aeros could have easily been the fallen angels which copulated with human women to create the Nephilim, but Nemmy's even more explicitly angelic "final" form could make the (theoretically) original Gestorumque fill those shoes. Being just another genetic crossing experiment by the Aeros would then retroactively turn Nephil/Ophion (and by consequence the entire race that was spawned in his wake) into a kaiju-human hybrid like General Gordon. Alternatively, the Ferox or the Antlanteans (or both) could have used Prime's remains for the same effect.
