Origins of the Valkyrie

When did the Valkyries first appear in Calpheon? Many of Calpheon's esteemed orders of knights often originated from remarkable heroic figures or were founded upon significant historical milestones/achievements. Hence the citizens of Calpheon weren't particularly skeptical about the Valkyries' inception. With the passage of time, much like the emergence of heroes or momentous events, they've come to accept it as just another part of their history.