Specimens are the "zombies" of the game who have been experimented by a group scientist from London, England. Horzine Biotech, a biotechnology company. Apparently, something went horribly wrong (as said in the Trailer) during the cloning experiments and the Specimens became extremely hostile, beyond their control. In the end, the lab got overrun and every living thing wiped out. In a matter of minutes, Horzine Biotech was already a living nightmare and eventually, the Specimens had already reached the city and destroyed civilization. These Specimens are not mutated physically but they are completely insane and lust for human flesh.The Specimens also tend to feed on dead Squad members which lead to the achievement, Dignity for the Dead (Kill 10 Specimens feeding on teammates).
Until today, the Specimens have only been known as "failed cloning experiments" while nobody knows the reason why the scientist (The Patriarch) is physically mutated and mentally insane as people believe that he is one of the surviving scientists from Horzine Biotech itself. To add more evidence to this, the Patriarch also claims these Specimens to be his "children" when the camera zooms on him on the last wave.
There are 8 types of Specimens which are very different from each other and have different special abilities each. Shooting or slashing off their heads will disable their special ability, this excludes the Patriarch as its head can not be shot off or slashed off while it is still alive.
The way the Specimens spawn are in a certain order. From the early waves, to the mid-waves, to the final waves and finally, the final boss.