According to the Enemy Guide, there're 4 metal regions in the game: A, B, C, and D. I think I've checked all the islands, and found only A and B regions.
Metal monsters from C and D can be occasionally encountered in other high-level regions (C in the final dungeon, D in the Skelly Adventure area), but their metal regions themselves elude me so far.
Could somebody help me with these two metal regions?
Metal regions C and D
Yep 
The metal region D is a small area enclosed by mountains SW from Dennev town ruins. Not sure about the metal region C, though.
Weirdly enough, I think the final metal monster from the D region shows up only in the D region itself, unlike all the other metal monsters, who can be encountered in non-metal areas. Unless, of course, it was RNG playing tricks on me.
The metal region D is a small area enclosed by mountains SW from Dennev town ruins. Not sure about the metal region C, though.
Weirdly enough, I think the final metal monster from the D region shows up only in the D region itself, unlike all the other metal monsters, who can be encountered in non-metal areas. Unless, of course, it was RNG playing tricks on me.