"'Chimæra. Animals on Lorien that could change their shape. They were called Chimæra.'" -Henri, I Am Number Four
Chimærae are animals native to Lorien. They were called out of hiding by the Phoenix Stones, and became the dominant fauna on the planet. Chimærae are able to morph into any animal at any time. Almost all of them were wiped out when the Mogadorians invaded Lorien. A handful of those who survived were captured by the Mogs two years after landing on Earth, until they were freed by Adamus Sutekh in 2011.
It was implied by Henri that Chimærae were not the only animals on Lorien. However, it is possible that the first Chimærae used their shapeshifting ability as an advantage in mating, and this led to most animals reproducing with Chimærae, spreading the morphing ability throughout the animal kingdom.
Both Hadley and Dust were thought to be the last Chimæra alive—before the end of The Power of Six, John and Number Six believed "BK" was the last Chimæra, and in The Forgotten Ones Adam believed that Dust was the last Chimæra.
Out of the fifteen that survived on Lexa's ship, apparently less than half of them survived. Olivia was killed by krauls, and many Chimærae died in Mogadorian experiments.
Demonstrated by Phiri Dun-Ra, the Mogs can paralyze Chimærae by changing the frequencies of their firearms, keeping them from shapeshifting.
When Chimærae die, they do not revert back to their "original" form, and instead freeze in the last form they morphed into.