Origin
The aquatic member of the Justice-9 from Earth-36, Mer-man is based on the Atomic Sub, the Big Bang Comics analogue of Aquaman.
Creation
Mer-man was created by Grant Morrison and Evan Shaner. His first appearance was in The Multiversity Guidebook #1.
History
The Multiversity Guidebook
Mer-Man appears in this issue as member of Justice-9, the Earth-36 analogue of Round Table of America (which is in turn based on Justice League of America). Earth-36 is an analogue of the Big Bang Comics Earth. Outside these two apppearances, there is nothing know about the character.