Arbitan (species)

The Arbitans, (PROSE: The Fishmen of Kandalinga) also possibly known as Marinians (PROSE: Doctor Who and the Keys of Marinus) and referred to as a "human race," was one of the "many races" native to Marinus.

Their technology reached its peak over two thousand years before the First Doctor visited, when they constructed and refined the Conscience of Marinus, living as pacifists for seven centuries until the Voords attacked. (TV: The Keys of Marinus)

The Doctor visited Kandalinga and encountered the Voord again, who'd been exiled from Marinus by the "Arbitans." The Doctor believed it was "many millions of years" since his visit to Marinus, and that "with the passing of the centuries the whole race had assumed the name of their great dead leader." (PROSE: The Fishmen of Kandalinga)

Behind the scenes
The name "Arbitan" was applied to the race in the The Dr Who Annual 1966 story The Fishmen of Kandalinga, set presumably millions of years in the future. In Doctor Who and the Keys of Marinus novelisation, published in 1980, Altos is described as a "Marinian," indicating these are the same species, although there are references to them as a being "human" and the existence of "many races" on Marinus, sometimes including the Voord.