Merfolk

Mermen were a race of aquatic beings that lived in the seas of Earth.

Biology
Mermen had a fish's tails from the waist down. Their top half appeared human. They had a second set of inner eyelids. Mermen were unable to breath for long periods on land. (BFA: Cryptobiosis)

History
King Neptune had mermaid servants. (TVC: Guests of King Neptune)

Graham Stevens claimed that a mermaid rescued his grandfather from the Marie Celeste. (NA: Cat's Cradle: Witch Mark)

The Mermen attempted to stay unnoticed by the humans on the surface. Two Mermaids were captured in fishing nets and taken by De Requin to be sold.

Amfetriti, one of the two, died. Her daughter Galliteya, the other captive, was looked after by Peri Brown. Nereus led his people aboard the Lankester where they killed De Requin. Galliteya was returned to them. Nereus promised help for the crew who had jumped overboard. (BFA: Cryptobiosis)

Myth
Mermen were also creatures of myth in Earth culture. Mermaids existed in the Land of Fiction, some whom were converted into Cybermen. (BFA: Legend of the Cybermen) The Eleventh Doctor called an alien doctor who stalked the Fancy a mermaid as she had the appearance of siren. (DW: The Curse of the Black Spot) Several other aliens were compared to mermaids, such as the Dax, (DYD: The Corinthian Project), the Meron and a race native to Gorgol XIII. (K9TV: Liberation)

The latter two species may have been the same.