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
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, 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)