Were the Praetorian guard part of the empire?

We know in Mando S3 there are a number of praetorian guard working with the imperial remnant, and given the dialogue, we know there are more out there than just the three we saw. Considering season 3 takes place roughly 4-5 years after the galactic civil war, is it plausible that the praetorian guard worked for the empire during that time frame? I tried looking at wookieepedia but there really is not much history on the praetorian guard or where they originate from.