For example, Growlithe is found in HG and Vulpix is found in SS. This means they are counterparts, right? But I heard that you can only find Mankey in HG. Then who is the counterpart to Mankey which can only be found in SS?
I can't find any counterpart to Mankey in HGSS, but it's counterpart in Red and Blue is Meowth.
Source: http://www.angelfire.com/ab/ultimatepinsir/counterparts.html