Well, i was doing VOG hard today and i received a fatebringer, but after moving n to the "exotic" chest in the Gorgons lair i found a Gjhallerhorn, now i'm not saying it is rare but due to it being a rocket launcher the chances of me using it a minimal, i was more exited about getting Fatebringer (and it was only my second run ever in VOG) but yes i do not think it is rare also seeing as how Xur will also sell it most times