Not a who but a what:
A Jenny Haniver is the carcass of a ray or a skate which has been modified and subsequently dried, resulting in a grotesque preserved specimen.
One suggestion for the origin of the term was “jeune d’Anvers” (French for Antwerp is Anvers), that is “young girl of Antwerp.” British sailors “cockneyed” this description into the personal name “Jenny Hanvers.”
For centuries, sailors sat on the Antwerp docks and carved these “mermaids” out of dried cuttlefish. They then preserved them further with a coat of varnish. They supported themselves by selling their artistic creations to working sailors as well as to tourists visiting the docks.