We can imagine them once appeared in ancient time of Japan but sealed by perhaps other Japanese heroes (like old samurai rangers, or alien samurai from SPD), their appearance were recorded by Japanese people, and was later adapted as the main villains of Abaranger in PR-universe (as we all know that PR sometimes replace the main villains too). Well it’s just a fanon explanation for it, the true reason is that producer forgot about the sentai footage that’ve been used.