The Alien is a size 6 ranged base creature added in Creature Chaos 2.8. Currently, it has the longest direct range distance in the game (reaching 45m at size 12) and is also tied with the Cheetah for fastest creature in the game per size.
The Alien has always been a popular base creature; in 2.82 and 4.2, the Alien had a powerful melee attack, Pack Hunter and Herding, making it a good counter to the slow, Herding-dependent Elephant combos of the time. Its speed and low stats made it an excellent level 2 base creature with Birth Defects, and could be combined with nearly any high-power, low-size creature to form a cheap, fast and spammable level 5 or powerful level 4.
As of 4.24, the Alien is now one of the most potent direct range units in the game, with incredibly high speed (allowing for easy anti-artillery micro) and a very long-ranged, Pack Hunter-augmented attack. Liopaliens, while not as powerful as in 4.23, are still a very formidable creature, sporting high defence, regeneration, high damage, high speed and a reasonable cost.
As with most high-speed critters, the Alien is quite fragile and has no powerful abilities aside from Pack Hunter (which only serves to balance its otherwise fairly weak ranged attack). Its size 12 actual health is below average at 1074.3 (712 HP, 25 D), making it easy pickings to powerful melee attackers if it is not well managed by the player.
The Alien is also no longer able to make useful level 2 and 3 melee units because of the obscene price of Pack Hunter at low levels. While the combinations are effective, the user must ensure that they have a very strong electricity income before attempting to mass alien-based combinations.
Undoubtedly, the best Alien combo is with the Liopleurodon (alien head, f/b legs, tail). The Liopleurodon gives it size 11, regen and high defence along with the option to take to the water to safely attack enemy creatures or structures. This, along with the 43m distance and 12 base damage (which increases to 17.6 with Pack Hunter's 40% bonus) ensures it can deal with most non-Charging melee creatures before they're even in range, and can take out focus-fire fragile Chameleon combos fast enough to minimise damage. Upgraded, its speed reaches 30L/26S, allowing it to kite charging units to some extent. With the Liop's high torso defence, its defence upgrade raises its actual health about 100 additional points compared to the Frowneleon.
For the most part, the Alien can be combined with nearly any large creature to create a very fast, very powerful ranged attacker which, in the right hands, is quite literally untouchable. Once its range distance goes beyond 40, you can safely attack just about any non-Chameleon direct ranged unit with near impunity.