The Fiji Mermaid is a famous hoax, sent into motion by famous hoaxer P. T. Barnum. He claimed it was a mermaid, and many people truly believed him. Actually, it was made from a baby monkey's torso sown to a fish's body, and covered in paper mache. New ones have been made, copying the original.
The Fiji Mermaid, and all those who followed it, resembled mermaids. There are some differences; the creature's torso looks more like a monkey, and the tail more like a fish.