The Dax were an underwater species from the extinct planet Daxus.
Biology
The Dax resembled mermaids, with half their body a fish and half a humanoid torso. They also had black membranes on their torsos and webbed hands.
History
The Dax were nearly wiped out by the Daleks, who destroyed Daxus. Some Dax had travelled to Earth, starting off the mermaid legend.
At an unknown future date, some Dax were living in an abyss near the Corinthian Project. They remained hidden, until discovered by the Doctor in a submarine. With the project about to blow up, the Dax helped to save the project. (DYD: The Corinthian Project)