NOT TO be pedantic, but mermaids — defined as half-human, half-fish denizens of the deep once believed to lure lovestruck sailors to their death — have been a staple of myth and legend for thousands of years. They don't exist.
There's a related tradition of more recent origin, perhaps dating back 1,500 years, which consists of manufacturing fake mermaid carcasses for public display using the body parts of dead monkeys and fish. The photographs you have just seen document one such artifact.
A similar specimen fabricated in Japan is believed to be 1,400 years old. Yet another, and by far the most famous, is P.T. Barnum's Feejee Mermaid, purchased secondhand by the great showman in the mid-1800s and exhibited throughout the United States as a sideshow attraction.
The glaring irony in all this mermaid fakery, with respect to the ancient tales on which it is based, is that the mummified specimens one typically finds on display are, without exception, hideous in appearance — "the incarnation of ugliness," as one American critic described Barnum's creature — while the classic mermaid of folklore and pop culture is invariably represented as beautiful and alluring. It is a discrepancy no one ever bothers to explain.
Update: These same images surfaced once again in early 2005 as part of a message claiming the mermaid carcass was washed up on a beach in Chennai, India by the Indian Ocean tsunami of December 26, 2004.
Sources and further reading:
Preserved Yokai of Japan
Cryptozoology Online, 29 June 2009
The Feejee Mermaid
Museum of Hoaxes
The Feejee Mermaid Archive
The Lost Museum
The Merman's Home Page
Last updated 11/27/10