Or it could be both, Faust could have pitched the alicorn twilight idea and then Hasbro decided to run with it and mandate it at the end of season 3 rather than a potential series finale, or it could have been planned by the writers far enough in advance for Hasbro to start making toys of it since the episode was supposedly written back in 2011.

Either way, the toys don't always follow the show or vice versa, we've still yet to see any mention or hint of princess Skyla despite existing as a toy for a year now, and we are only now starting to see models of background characters that have been hanging around since season 1.