Maybe the aggressive nanomachines that make up Technovore?
Technovore was created when a team of scientists working aboard the orbiting Stark space station accidentally created a microscopic artificial life form. Their company had 'rented rooms' as it were, but when months of research yielded no results, an attempt at forced natural selection had indeed produced an autonomous nanomachine - a self-replicating micro-mechanism which voraciously consumed anything it encountered and partially replicated its macro--structure, learning at tremendous speed along the way.
It devoured the staff, learning to build humanoid forms, then consumed machinery and the ability to form itself into weapons; but its most dangerous quality remained the fact that any contact with it was lethal, since its microscopic components were still parts of the whole and would quickly consume and assimilate anything they touched.
Iron Man did what he could to contain the Technovore to the space station and send it out into deep space, so far away from Earth it could not return under its own power, but in the end the creature devoured the entire station; ultimately, it was destroyed when Stark managed to activate the station's auto-destruct and it was consumed in a fusion blast.
While Technovore was an amoral, infant life form, driven only by voracious hunger for a form of fulfillment which was hopeless since it did not know what it was looking for (namely, an identity of its own), it was insanely dangerous; it proved capable of assimilating anything and it would most assuredly turn on the Guyver, a Zoanoid or a Zoalord. Biology being the basis of all their abilities, it remains to be seen whether they could resist its onslaught; unlike Aptom, Technovore did not bother with infiltrating and rewriting DNA, it gobbled cells right up, analyzing them as it went; and it did so in seconds.