The Arbitans were a humanoid species native to Marinus. (PROSE: The Fishmen of Kandalinga) They were pacifists until the Voords forced them to fight. They tried to defeat the Voords by fixing the Conscience of Marinus. (TV: The Keys of Marinus)
When the Doctor visited Kandalinga, he learned from the Voord that "with the passing of the centuries the whole race had assumed the name of their great dead leader" — Arbitan. (PROSE: The Fishmen of Kandalinga)