I remember watching one of those horror anthology shows back in the
60’s where a professor/scientist was teaching a class of medical students.
On a table is a mermaid. The professor says he has invented a cure
that will give the mermaid human legs. He gives the injection and a few
minutes later he throws back the bottom half of the sheet to reveal
beautiful human legs. The class congratulates him and then he takes
the rest of the sheet to reveal A FISH HEAD.
I have no real idea about that show, but it sounds amazingly like something Ernie Kovacs might have done. In case he’s not familiar, check out Ernie Kovacs - Wikipedia