Inuk

The Inuk were artificial beings created by the Thanatos to protect itself, acting as its antibodies. They resembled humanoids with gray skin and large eyes, although they could also transform into large, crustacean-like forms with multiple claws.

When Colonel Snathe of the Betrothal of Sontar and his Sontaran crew arrived on Serac in search of Thanatos, they believed the Inuk to be the native life forms, and during their long stay on the planet took to hunting and killing them for sport, although the Inuk secretly killed Sontarans as well. When the Doctor and Snathe found Thanatos, the Inuk attacked them, pursuing them deeper inside the device. After Thanatos judged Snathe as unworthy, the Inuk captured him and threw him outside during a Silverfire, killing him. (DWM: The Betrothal of Sontar)