This grotesque object is the "Feegee Mermaid," another of Barnum's longest running and most popular attractions. The image above, created by The American Social History Project, is a computer rendering of the original object, which still exists. Barnum bought it from a fisherman who had stitched it together out of the bodies of a fish and a baby orangutan and the head of a monkey. It probably fooled only the most credulous viewers.
Barnum always insisted that he was not a con man: he was what he called a "humbug." He never denied he was out to fool people, and he argued that what he sold was spectacle, illusion, and the opportunity to figure out the deception for your self.
This is another image of the "mermaid," from a lithograph done in the 19th century. Barnum may have made more than one mermaid, and it also seems that others exhibited "mermaids" of their own, all claiming to be Barnum's original.