In "Lost Girl", Rumple and Charming knew about Excalibur aka the Sword in the Stone
When Snow showed him the FAKEcalibur, he destroyed it
What if the real sword was still in the stone ?
If the sword had been pulled out, Rumple must have said to Snow that someone else had done that, before turning it to dust
Maybe there was no King Arthur at that time. Maybe there was just a guy named Arthur
I theorise that Arthur had ruled Camelot several years after 3x02 and before the Dark Curse was cast, or he was crown after the Curse was broken by Emma
P.S : When did Lancelot show up ? Before or after Snow got cursed ? I forgot everything about season 2 :(