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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 and later the Sentinels. Nathan Hale and Jordan Shepherd were injected with a special strain of Pure Chimeran DNA; Hale became semi-immune but later succumbed to the virus. Shepherd was mutated into the Chimeran entity known as Daedalus and took 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. However, their plans are irreversibly dashed by the actions of Joseph Capelli, who destroyed the wormhole to the Pure Chimera's planet and cutting them off from Earth.
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 within 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 New York City, inside one of the Chimeran structures. If look closely with the Deadeye or Marksman (level 2) you can see them in a curled up position in blue "containers". Despite how big they are in Resistance 2, the Pure Chimera in Resistance 3 are considerably smaller compared to a human.