It is possible to organise the Aliens in two main categories, organic and mechanical, and in each of the two we can find a couple of different enemies. The basic ones are super aggressive but come unarmed, relying only on kamikaze attacks, chasing players regardless of their position on the map.
The other type presents a gun (or two, in the enhanced version), but has a determined aggro range and wont attack unless you come too close.
Organic Aliens Edit
Mechanical Aliens Edit
These alien ships are more common and spawn in every map after a determined amount of time. In this category falls also a special rare ship that transport a cargo with 3 Auto Firing Drones power-ups inside, that can be collected upon destruction.