Progenitor Mimic

Any enters-the-battlefield abilities of the copied creature will trigger when Progenitor Mimic enters the battlefield. Any “as [this creature] enters the battlefield” or “[this creature] enters the battlefield with” abilities of the chosen creature will also work.

You can choose not to copy anything. In that case, Progenitor Mimic enters the battlefield as a 0/0 Shapeshifter creature and is probably put into the graveyard immediately.

If Progenitor Mimic somehow enters the battlefield at the same time as another creature, it can't become a copy of that creature. You may choose only a creature that's already on the battlefield.

If the chosen creature is a token, Progenitor Mimic copies the original characteristics of that token as stated by the effect that put it onto the battlefield. Copying a token doesn't make Progenitor Mimic become a token.

Progenitor Mimic copies exactly what was printed on the original creature and nothing more (unless that creature is copying something else or is a token). It doesn't copy whether that creature is tapped or untapped, whether it has any counters on it or Auras attached to it, or any non-copy effects that have changed its power, toughness, types, color, or so on.