In Normal and Nightmare modes the Zombie Queen may lay a Giant Egg some time during her battle. This is a lesser Tartarus and lacks many of its abilities. The egg itself is strong, but vulnerable, and can be killed before it hatches with liberal amounts of firepower.
The Giant Egg in the airlock, the same one fought by Easy Co, will slowly grow throughout the course of the Apollo Security Team Campaign. It begins very small but will be nearly full grown by the end of the campaign.