Edward Rodley described fejee mermaids as sailor-made artifacts:

Sailors also made Jenny Hanivers.  These were guitarfish (elongated
relatives of sting rays) cut, twisted and then dried - they look like
little demons.  Modern sailors still produce these (but without the
personal name) and sell them as curios.  The references I've read
indicate the origin of the name Jenny Haniver is not known.

Linda Wilson
Visitor Studies and Evaluation
Shedd Aquarium
Chicago, IL