The Unknown Alien serves as the final boss of both Metal Slug Mobile 4 and Universal Metal Slug: Tank Conspiracy. Its origins are unknown, but the Rebels are seen possibly attempting to destroy it when the player enters the room.
The Unknown Alien has three attacks. When the alien lifts its head, it will open its mouth and fire a laser. Its stomach can either produce organic, timed bombs or a short-range fireball. Once the alien received enough damage, an eyeball from the top-left corner of the room will open its eyelid and fire various projectiles.
Its weak point is its head. If hit enough times it will die, and the alien's lair will be destroyed.
- The boss room is similar to the Organic Fortress stage from Gradius Gaiden.
- The boss itself resembles the Gofer in Gradius II and Mother Brain from the Metroid series.