nahh.. I don't think he's a real goblin...and based on the latest chapter, they said about how every human beings has different face/appearance because that is their identity. so, because he has the same appearance as the first Prince they just assume the Prince is a monster in disguise
the saddest thing is how everyone one treat him like a monster... even the first consort/concubine he has doesn't want to live with him and would rather end her live
So twins were considered a bad omen, even in real life back in history so I believe that’s where the “goblin” (evil) comes from.
So is he really a goblin or do they call him that because they never saw twins before?! I really like this story i just hope the mc will be badass in the future