Nyacos eggs are rumored to be laid by the ancient Arkian gods and then placed in the Cave when no one is looking. This is how people explain their incredible rareness.
Nyacoses of the Asteroid are thought to be divine creatures from the Heavens. Only the ancient Arkian gods know who are good and strong enough to receive a Nyacos. If you are one of the lucky ones, you will be in for quite a challenge because Nyacoses are among the most proud and stubborn of creatures in the known universe (difficult to train) and also the most difficult species to hatch and evolve on the island.
Nyacoses, when they are first born, require and need the ability to roam free away from their owners. In fact, a newly born Nyacos needs to ascend into space (outside of the atmosphere of Ark) and gaze into the stars, perched on the Asteroid itself, to understand themselves and where they come from. A Nyacos who is deprived of this experience will simply not evolve to the next stage — they will not "know themselves" fully.
Once a Nyacos is given its cosmic perspective, it may return to you. Some will permit you to train and ride them while others will not. If you attempt to mount a Nyacos that wishes to remain wild, it will likely kill you.
These creatures are worthy and deserving of the utmost awe and respect.