Tanu and Firvulag
The exotics inhabiting the Pliocene Epoch, despite being separated from the appearance of humans on Earth by millions of years, closely resemble the Tuatha Dé Danann and Firbolg of Celtic Mythology.
The exotics are known as the 'Tanu' and the 'Firvulag', and together constitute a single dimorphic race. The Firvulag are the 'metapsychically operant' members of that race, and the Tanu are the 'metapsychically latent' half. However, the majority of Firvulag have only weak mental powers, whereas the Tanu wear torcs, which are also mind-amplifying devices to allow use of their mental powers. The Tanu are generally much longer lived than the Firvulag. The four books of the Saga of Pliocene Exile abound with Tanu who are more than a thousand years old, who were not born on Earth, and who are called 'first comers' because of the fact. Examples of 'first comers' include King Thagdal, Celadeyr of Afaliah, and Dionket Lord-Healer. The Firvulag are not usually as long lived, although they have a few first-comers of their own (King Yeochee and Palloll One-Eye among them), but are physically hardier and more resistant to Earthly radiation than the Tanu.
Tanu and Firvulag are sworn enemies, with each race routinely attacking the other. The only exceptions to this are in the month before and the month after the ritual Grand Combat, a gladiatorial combat that pits the Tanu against Firvulag. These two months are called The Truce.
Firvulag babies are frequently born to Tanu mothers carrying recessive Firvulag genes. These babies are cared for until they can be handed safely over to the Firvulag. Firvulag never produce Tanu babies.
Both Tanu and Firvulag can be killed by objects made of iron, which they call "blood-metal." Its use in weapons is forbidden by their battle code. Since the exotics are otherwise extremely difficult to kill, some humans take advantage of this weakness.
Read more about this topic: Saga Of Pliocene Exile, Pliocene Races