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The "Pure Chimera" are the original species of the Chimera, and are not created by parasitism, as standard Chimera are.
It is presumed that the species originated from another world and occuppied Earth more than 60 million years ago prior to the existence of humanity. At some point in time they entered into a violent conflict with an unknown alien race which inevitably forced them to completely abandon Earth and promptly vanish from history.
Before having left Earth it is believed that the "Pure Chimera" built the Chimeran tower network and left them buried. It would be millions of years later, well into the twentieth century, that the species plotted to reclaim Earth. Apparently the Pure Chimera were strongly implied to be at a severely weakened state or presumably on the brink of extinction by their unknown enemies and, as a sort of desperate attempt, sent the Chimeran virus to Earth in 1908.
It is unknown how, but during the Chimera's invasion of Russia after 1908, Dr. Fyodor Malikov was able to collect specimens of Pure Chimeran DNA, which he would use to create the Cloven as well as to inject into Nathan Hale (who became semi-immune but later succumbed to the virus), and Jordan Shepherd, who would mutate into Daedalus and take sole command of the Chimera world-wide.
By the time of the Chimera's near-dominance over Earth in 1957, the Chimera were making preparations in terraforming Earth into a hospitable environment for the Pure Chimera's arrival to the planet.
The Pure Chimera are first mentioned and seen in Resistance 2, in Bryce Canyon. Their preserved bodies are curled up and encased in transparent egg-like containers with the Bryce Canyon Chimeran tower. The creatures seem to be about 12-13 feet (3.6-3.9 meters) long while uncurled, but this is not certain. They resemble long, segmented armored worms, with multiple eyes on their heads and sharp, shearing teeth. Their heads are similar in appearance to those of Hybrids.
A fossilized skeleton of a Pure Chimera can also be found in the mines of Mount Pleasant in Resistance 3. They also appear in the level New York City in one of the buildings of the chimera. They are curled up on one of the containers (blue) if you look closely with the Deadeye or Marksman (level 2). Despite how big they are in Resistance 2, the Pure Chimera in Resistance 3 is much smaller than a human.