A derg is a dog. Dergs are more often eaten than kept as pets, and many dergs are seen in settlements roaming and scavenging. They are prized by the poor as being preferable to rat meat. Those that are kept as pets are often stolen, so owners usually keep a close watch on them.
In Outrider (book), Nine's pet tekira, Nibble, is mistaken for a derg, as people of the Verges are unfamiliar with the Ildorian creature.