I do believe cougar and puma are the same thing since their Latin name is puma concalor.
In any case, panther isn't a species. Panther is a name for a coat colour. So if you were the puma concalor variant of panther, nothing but coat colour has changed. There are also leopards and jaguars that could have been what the panther was, these species are very different from eachother in behaviour iirc.
Edit: since panther is usually confused with the puma concalor, as it has many informal names, (cougar, mountain lion, screamer cat, panther), i suspect the initial creators of nwn did intend it was puma concalor.