You can use the Freezing feature to store the mutants that you don’t use quite often. That way, you’ll free up some space on your rooftop and will be able to place your favorite creatures.

To freeze a mutant from your rooftop, click on it and select “Freeze” at the bottom.

You can also click on the ice palace-looking building, and then click on “My mutants”. There, you will be able to freeze/unfreeze your creatures.