The Dnyarri were an extremely ancient and evil race who could control the minds of others. Their discovery by a young Ur-Quan scout led to the quick collapse of the Sentient Milieu, to the thousands of years of Ur-Quan slavery and their race´s eventual splitting in to the Kzer-Za and the Kohr-Ah.
The Dnyarri favored the Ur-Quan over the other species of the Mileiu, due to their weak resistance to mind control. They used the Ur-Quan to wipe out many member races of the Mileiu over time. During this period, the Dnyarri re-engineered the Ur-Quan DNA into two types, the green (Kzer-Za) who were the political leaders and scientists, and the black (Kohr-Ah) who were the builders, warriors, and general "do"ers.
The Ur-Quan leaders Kzer-Za and Kohr-Ah learned to resist mind-control through continuous self-torture, and used their freedom to broadcast the message to the rest of the Ur-Quan, who similarly inflicted pain and agony upon themselves to relinquish their minds from the Dnyarri control. The freed Ur-Quan slaughtered the Dnyarri en masse.
Death wasn't a good enough punishment for the Dnyarri in the eyes of the Ur-Quan. As they had been engineered, so too did the Ur-Quan re-engineer the Dnyarri into sub-sentience, making eternal slaves of them, using them to communicate for them whenever they communicated with a species they felt was less advanced than they were (which was often). These Talking-pets are used aboard all Dreadnought and Marauder vessels.
The Arilou describe the Dnyarri as the most evil race known to this galaxy.