Dnyarri
The Dnyarri were an ancient, extremely evil race who could control the minds of others. They resembled large, brown toads with a grossly pulsating skull and sinister looking face. Their discovery by a young Ur-Quan scout allowed the psychic Dnyarri to quickly dominate the Sentient Milieu. The Ur-Quan were enslaved for thousands of years under the Dnyarri's yoke, and were eventually split in to the Kzer-Za and the Kohr-Ah.
The Dnyarri favored the Ur-Quan over the other species of the Milieu, due to their weak resistance to mind control. They used the Ur-Quan to wipe out many member races of the Milieu 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.
After the Dnyarri had grown complacent, and had given some time free to the Ur-Quan, most likely due to their acquired laziness, the Green Ur-Quan researcher Kzer-za found out that the Dnyarri had to disconnect from a slave's mind if the slave experienced extreme pain. Kzer-za broadcast this discovery to the rest of the Ur-Quan, who started inflicting 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 recently found a Talking Pet in the wreckage of a Dreadnought at Alpha Pavonis VII. They took it to the Umgah at Beta Orionis to heal it back to health. The Umgah, unfortunately, nursed it back to full, sentient, evil health, complete with its mind control powers.
The Arilou, Chenjesu and the Ur-Quan describe the Dnyarri as the most evil race known to this galaxy. The Taalo fled to another dimension because of the Dnyarri, at least according to the Orz.