Apparently it's a mermaid. I encountered it at Hartlepool Museum. In Mortlock, Scrabsnitch is not above stitching together bits of deceased animal to make 'curiosities' that will draw the punters in and here is a real life example. The mummified remains of a monkey were stitched to the body of a fish to make the mermaid.
In the 1850s, people paid good money to see things like this. Many folks didn't travel beyond the boundaries of their own parish and anything like this, rather hideous but strangely interesting would have been a real draw. Even now, although it was one exhibit in a whole cabinet full of interesting artefacts, this one sticks in my mind. Maybe I should get out more often!