There are mermaids
in at least one of the worlds of Series Five
of the Related Worlds
; some of them appear in The Lives of Christopher Chant
They have greenish hair and greenish skin, and speak in lisping voices. They have big silvery tails instead of feet. They live in tribes. Christopher seems to like them and is appalled when he finds out why they have been missing as of late.
The flesh of mermaids is highly prized on the magical black market.