You may have Progenitor Mimic enter the battlefield as a copy of any creature on the battlefield, except it has "At the beginning of your upkeep, if this creature isn't a token, create a token that's a copy of this creature".
The table explained to me that because Niv-Mizzit is a legendary creature, Progenitor Mimic couldn't continue copying the creature. So the next turns only one Niv-Mizzet copy remained in play under my control. And we left it at that.
Now post-game I'm wondering, which permanent was actually on my battlefield? Did the original copy of Niv-Mizzit with Progenitor Mimic remain (destroying the token copy each turn), or did the token copy remain because the original copy got destroyed when the token copy came into play with the same name?
I'm still pretty new to the game, so it would be great if anyone could tell me what happens exactly when you play Progenitor Mimic to copy a legendary creature. Please include the turn of playing it, and the next one (or more turns, if anything changes).