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 also contend with the Mire Beasts and a Dalek invasion when they landed their time machine on Aridius in search of the First Doctor and his companions. They chose to collaborate with the Daleks and surrender the Doctor and Barbara Wright.

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

Behind the scenes
According to non-narrative information in DWBIT 41, 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.

They also have the distinction of being one of the only few "classic" species from the original series to only appear in one story in one medium as of 2018.