Well, the chick grew up into a chicken, but you’re still not sure how the egg got into that cave...

Mutant Space Chicken believes she is from space. But because there isn't much information about where she is from, it continues to remain a mystery. All Mutant Space Chicken knows is that her mission is to populate Earth with more mutant chickens. When it's dark she creeps into a chicken coop and lays an infected egg. She cracks it open with her malformed beak, spills the goo contained within upon the normal eggs and then creeps back out into the darkness. When it's time for the eggs to hatch the misshapen baby chicks peck furiously at the shell so they can escape. Afterwards, they run into the forest and hide. As of right now they are still waiting for the right time to take over the planet Earth.