It was mentioned that the Cougar was a totally seperate plane from teh Panther but shared the same designation. This is very common among all countries who develope aircraft and must rely on political entities to provide funding for such. About the only thing common are the parts of the cockpit, the tail, and the guns. The Germans did this with the JU-188 etc, and other aircraft since Hitler wanted to concentrate on production of AC they already had on the line. The designations are often a result of political considerations, like the German MP-44 being called a submachine gun (machinenpistole) rather than a new rifle; Adolf had already complained about the number of new weapons that were messing with production efficiency and logistics.
The USAF and USN has done this many times, including today, with the F-18E/F SuperHornet. That thing only has a basic layout in common with the original legacy Hornets,and is much larger and has almost no commonality, maybe the landing gear. BUT, they had to get it past a congress that knows little about the subject matter, so the Navy got a new strike fighter that was a totally new design.
The USAF F-86D was originally the F-95, sharing only the basic wing and landing gear with the F-86. However, despite the huge defense expenditures of the time, the USAF was developing a large number of new fighters (the century series), and didn't want to push it, so put the D designation to make Congress and the media think it was a variant of an existing plane, which it actually wasn't.