KURASHIKI, Okayama Prefecture—Researchers using high-tech equipment have reached a conclusion concerning a mysterious and creepy “mermaid mummy” long kept at a temple here: It is mostly fake.

The creepy mummy is 11.5 inches long and closely resembles the "Fiji mermaid," which was an elaborate hoax concocted in the 1840s by a showman named P.T. Barnum. For the first time, researchers ...