Firstly, Glasspool Mimic does not goal; quoting from its Oracle textual content:

You might have Glasspool Mimic enter the battlefield as a replica of a creature you management, besides it is a Shapeshifter Rogue along with its different varieties.

You don’t have to decide on the creature it’s copying till it truly enters the battlefield. You can select any creature you management at that time, and your opponent doesn’t get to reply to the selection as this doesn’t use the stack.

If you don’t have any different creatures accessible when the mimic enters the battlefield, it enters as a 0/0 creature and can be moved to the graveyard the subsequent time state-based actions are checked except there may be another impact growing its toughness.