Aridian

The Aridians were a piscine humanoid race native to Aridius.

Biology
Aridians had silvery skin, crested heads and fins along their spines. (TV: The Chase)

Culture
When the First Doctor encountered the culture, it was clearly in decline, having all but lost the evolutionary war on Aridius to the Mire Beasts. The Mire Beasts had virtually uncontrolled growth, which had pushed the Aridians to a tiny, hidden stronghold. The Aridians were led by a ruling council of elders, whose orders could not be challenged by their juniors. (TV: The Chase)

History
The Aridians originally lived in an underwater city, but the oceans slowly dried up and turned into deserts. Not only did they have to adapt to this new climate, but they also had to contend with the Mire Beasts and a Dalek invasion whom had landed their time machine on Aridius in search of the First Doctor and his companions. The Aridians chose to collaborate with the Daleks and surrender the Doctor and Barbara Wright.

Barbara, however, still tried to save them from the Mire Beasts, but was pulled away by the Doctor. (TV: The Chase)

Many years after their encounter with the Doctor, the Daleks attacked Aridius, as it reminded them of their failure to catch the Doctor. After the destruction of the Mire Beasts, the Aridians were left undefended and easily exterminated. (PROSE: The Sagarro Desert Storm)

During the Last Great Time War, Romana II was aware that Aridius had been invaded by the Daleks. (AUDIO: Beyond)

Alternate timelines
In the Warrior's universe, the Warrior met with an Aridian by the name of Chooban. Aridius had been ravaged by Dalek invaders and was slowly drifting closer to its sun, causing the evaporation of surface water and the extinction of species. (AUDIO: Dust Devil)

Behind the scenes
The Aridians had the distinction of being one of the only few "classic" species from the original series to only appear in one story in one medium until the release of the Doctor of War audio series, in which they made their first appearance since 1965.