never heard of such a fish. there are many, many different species of groupers (marine fish) ... but butterfly grouper is not among them, there are marine butterfly fish (Chaetodontidae), and there's the African Butterfly fish -Pantodon buchholzi which is freshwater. i've seen the name "butterfly goby" given to fish that are actually Vespicula depressifrons. other "common" names for this fish include "freshwater Waspfish" and "leaf goblinfish". Try a Google search on "vespicula".
and finally, there's the Butterfly Perch - Pseudochromis paccagnella
it sounds like a 'made up' name that was given to the fish in question by the "know nots" at Petland and is not an actual fish name.