Finally got my hands on Merdude from the original run of TMNT action figures! Another flea market score, this guy is missing his accessories but is in otherwise mint condition!
I've only ever seen Merdude at the flea market once before, many years ago, and he was in rough shape so I had to pass.
Ironically, I just caught a recent episode of the modern Nick TMNT cartoon, and the Turtles were on some planet with fish creatures that looked a lot like Merdude. I have no idea how Merdude worked into the original TMNT storyline, but I'm sure I could easily look it up if I wanted to.
Regardless, I'm pretty confident he's one of the interdimensional characters and not an Earth mutant creature. Feel free to correct me if I'm wrong.