The mecha egg is a Mecha Swarm replication of zerg cocoons created by Egon Stetmann during the End War. They are the shell defending a mecha larva as it changes into one of the many mecha zerg strains.
The mecha egg is the pupal form of the mecha larva for Egon Stetmann in Co-op Missions. They function identically to the zerg cocoon, and allow him to spawn his base units. They are mechanical rather than biological.