Progenitor Mimic can be cast as a copy of cards like Mutavault and Simic Keyrune whilst they're in their creature state. When it does so, it can become something other than a creature: a land, an artifact, etc.
However, its copy ability states:
At the beginning of your upkeep, if this creature isn't a token, put a token onto the battlefield that's a copy of this creature.
Suppose my Progenitor Mimic becomes a Mutavault^. At the beginning of my upkeep, it will be only a land. Will Progenitor Mimic create a new Mutavault token, or is this prevented by the fact it isn't a creature?
^ For those who may not know, it will copy exactly what's printed on the Mutavault card and be a fresh Mutavault, not a 2/2 creature, because of how copy works. You can check the 4/15/2013 rulings on Progenitor Mimic.