Actually, as PJ's link says, I too think its an interesting taxidermical work. You take a large fish, and gut it, you leave the skin attached to the tail. Next, you add two fins from a fairly large tuna.
The head... Uhmm... no idea, but possibly a monkey skull with shark teeth? Then coat everything with fish skin/skales....
Voila, a sea-monster! It would need a very able taxidermist though to do it properly...
EDIT: after a quick search on snopes, it seems that the person who did this:
http://www.museumofhoaxes.com/hoax/weblog/permalink/mummified_sea_monster/
is the same person who did the mermaid (in the horse-head thing, he got asked a question and answered with:
Q: This is incredible! How do you make these?
A: I will be listing a 5 foot long REAL mermaid next...come back to see her!
Second edit, with another thought about the skull... Teh skull is definetly from a land-dwelling creature. Look at its eye-sockets. They face forwards if the "thing" stood up. Fish eyes are on the sides of their skulls (bar some flat fish who live on the sea-floor), but either way, they face forwards when they are horizontal... not vertical. Hence, a land-living skull... The teeth... hmmm I'll have to research those
Third edit... the teeth... could be a shark... the Mako Shark teeth are similar, but the jawbone would need to be a little longer to cover the roots...
Last edit... i promise!
http://cgi.ebay.com/Authentic-Organ...DVWQQrdZ1QQcmdZViewItem?hash=item300034045814
theres another one by the same seller... This one definetly made with shark fins/tails and monk-fish fins/tails. Not that great as the mermaid though...
Michele