I’ll just quote the Wikimedia Commons text on this one. It tells a better story than I could.
A relatively benign merman complete with scales caught in the Baltic Sea in 1531, according to Johann Zahn’s sources.
In: Specula physico- mathematico-historica notabilium ac mirabilium sciendorum by Johann Zahn. published 1696 at Augsburg, Germany.
Library Call Number Q155 .Z33 1696.
The caption reads:
“Vir marinus episcopi specie
An: 1531 captus
im mari Baltico.”
Image ID: libr0081, Treasures of the NOAA Library Collection
Photographer: Archival Photograph by Mr. Sean Linehan, NOS, NGS
National Oceanic and Atmospheric Adminstration (NOAA), United States of America
Secondary source: http://www.photolib.noaa.gov/library/libr0081.htm