They are one and the same, right? Or is there something I don't know? (Just started watching)
If I understand right, Proto Mercenare outright demonically possesses the king, and while using the king as his vessel, goes by King Mercenare. Then when the king is released from his control, he takes his true form (or possesses Bel, who swallows the last fragment of King Mercenare).
But Wikipedia lists them as separate characters, and they have separate pages both here and Villains Wiki.
Is Proto Mercenare like Moo from Monster Rancher, possessing a host, simple demonic possession? Or was King Mercenare a separate entity created by him corrupting the king? It sounds like the king's consciousness was trapped inside this monster meaning the king and King Mercenare are separate entities. Would this then be like Jecht becoming Sin in Final Fantasy X, with Yu Yevon as Proto Mercenare?
(Maybe it'll make more sense once I seen the last few episodes?)