The Terraberserkers from the Kodonian Belt are a sapient alien species known to have been present at the Festival of Offerings in the rings of Akhaten. Upon spotting one, the Doctor remarked that "you don't see many of them around anymore".
They have lavender skin and facial jowls, but much of their face was concealed by a hood.
It is likely that the 'Cadonian Belt' refers to the asteroid remains of their previously destroyed homeworld. This would explain why the Doctor said this species is rarely seen anymore.
- Doctor Who, new series' season 7 - "The Rings of Akhaten"