Overspawn

In X-Com: Apocalypse, creatures referred as Overspawns are gargantuan, mindless alien creatures dropped by extradimensional aliens on last remaining human city on Earth, Mega-Primus.

Overview
Overspawn is massive. It is the size of a small building, with multiple appendages with hardness and strength to crush and destroy any existing material in our dimension. It strikes anything and everything in it's path with little consideration for it's own safety.

This alien bears semblance especially on genetic level to megaspawn, another alien with the functionality of a living tank. As with megaspawns, it is highly unlikely that overspawns can be born in our dimension, as a single chrysalis cannot physically develop into a creature of this magnitude. Most likely they are biologically engineered in the same organic factory as alien weapons in their home world.

Role in invasion
Overspawns are first and foremost a terror unit, and quite adept at that. Being a creature of such enormous proportions, there is no question that aliens are no longer attempting to hide their invasion. When the alien mothership drops one on Mega-Primus, it will attack anything and everything it can reach, wrecking buildings and armored vehicles it can catch.

Overspawn is the definition of brute force solution. It seems quickly and shoddily constructed and as it bears no ranged weaponry or the sophisticated nerve-endings of megaspawn that allow for weapon links, it is likely that aliens have not had the time to evolve overspawn to the apocalyptic creature it might otherwise be.