Mancubus: not natural fauna
Pictures coutesy of the UAC Survelliance Group
The Mancubus was not found to occur naturally among the fauna on the other side of The Portal, and no specimins were observed in the environment by the expedition team. There was also no organism found with enough genetic similarity to the Mancubus to have been the likely genetic base. This was further evidence to support the current consensus, which is: The Mancubus was the creation of a (sic) intelligent alien or alien race, and Human genes appear to have been used as a genetic base. This conclusion is based upon studies of genetic material from Mancubi obtained shortly after the Portal Invasion. The Human genes are so heavily modified with gene insertions from one or more of the species within the Bipedfuga family that the resulting organism is not and cannot be considered human, it is a wholly different life form.
It is believed that the Mancubus was the first and only successful alien creation of an invasion-oriented monster that used human genes as its base. Furthermore, the resulting organism had tremendous physical problems and chemical imbalances that were not evident in the other invasion-oriented genetic creations, and is the least viable of the genetic creations observed in the Portal Invasion. It is theorized that either the alien(s) were not done refining this life form creation, or something about the human genes being combined with Bipedfuga genes had proved to be difficult for them to make work well.
Mancubi were always seen wearing a firebomb launcher on each arm, along with small tanks with the propulsion agent for the firebomb launchers on their backs. This equipment was not surgically attached, and could presumably have been removed and replaced by the alien(s) that maintained it. Mancubi appear to have been incapable of removing this equipment themselves.
Most of the information we have regarding Mancubi comes from two specimins which were found trapped in a locker room in the UAC facility shortly after the Portal Invasion had been put down. These individuals were captured for scientific study, but died within a few days of their discovery.