Invaders
Invaders is the only term the quasi-alien entities who appeared on the moon one day that can qualify. Appearing from out of practically nowhere, they forced humanity into an atrocious war that could only be won by the Super Robot Army and its Vanguard, the invincible Getter Robo.
History
Very little is known about the Invaders prior to their appearance on the Earth's moon, but there is a very good chance that they have been active and among humanity for quite some time. They appeared one day from a discarded test Getter machine, using it as their impromptu mode of transportation to take over a space exploration colony, forcing humanity to scramble a way to defend themselves, and forming the invincible Super Robot Army, led by Professor Saotome's Getter Robo. After the Moon Wars were over, and the surface of the satellite was scoured clean of the Invaders, it was finally over and humanity could rest again.
Except that it wasn't. Years later, after the Getter team had scattered to the winds due to personal reason, the true extent of the Invaders' abilities was learned. They had in fact been on Earth for some time and attempting to sabotage their enemies in ways far more insidious than previously imagined. They caused the death of Professor Saotome's daughter, Michiru, and their leaders had taken up positions in the UN's security council. They were far from mindless. They caused a nuclear missile to detonate a Getter Reactor, covering the planet with Getter beams and rendering 60% of it uninhabitable.
Abilities and Zoology
Invaders were shapeshifting life forms which could originally change into any form they desired, allowing them to be both incredibly sneaky and incredibly strong as the case may be. Their shapeshifting was such that no true 'one' form could qualify as their real form. An Invader which was fighting on the ground could sprout wings and spontaneously take off to fight in the sky with ease, or grow fins and tentacles to match blows with any underwater opponent. Their strength is directly proportional to their mass, and the most deadly ability exhibited by Invaders in the Moon Wars was the ability to merge together into a more solid and powerful fighting form.
With their merging with Getter Rays, however, Invaders became parasitic with life forms and symbiotic with machines. Their mass/strength ratio was suddenly unimportant, allowing a building-sized Invader to masquerade as a puppy, or a human-size Invader to match blows with a giant robot, as well as generate smaller invaders in mass greater than its own body. Many Invaders fused with machines they defeated, adapting their abilities with their own shapeshifting powers, and adding weapons where necessary.
The most insidious ability of the evolved Invaders has to be their ability to infect and possess human beings. Any human who has had direct contact with an Invader may have become infected, making their defeat itself an exceptionally dangerous process. As with their earlier abilities, an infected human can shapechange and grow to several times its normal mass if the Invader inside it wishes to. Those who do this have usually mastered.. or will almost immediately learn, human speech and mannerisms. Note: For all description where the singular is used, assume there is more than one Invader (see below) inhabiting a body or forming a single whole.
Organization
The Invaders are a hive mind species. What one learns will be learned by every other Invader they merge with, making their networking abilities unparalleled. All Invaders in more or less direct sight of each other can know and see what every other Invader there does, although it is still possible to get the element of surprise, as the 'transfer' is clearly not instantaneous. They can merge together to form a whole which is quite capable of independent action, and generally the Invaders seem smarter the more there are in the mass, or at least more clever.
There are Invader 'Queens', if such a term is to be used for them. These are clearly the leaders and ostensibly birthers of the Invaders. Doctors Stinger and Coen are both able to generate more Invaders and can control them with just their will. Treating them as a half-solid, parasitic insect race is as close a parallel as we can make, but the Invaders are clearly 'alien' in everything they do.
The true Origin of the Invaders actually lies with life itself. More biologically than conceptually, the Invaders are the anti-thesis of life. They grow and evolve but all they do is imitate and steal. They serve no purpose in any food chain but their own and they are basically entropy to life itself, both conceptually and officially. Their true origin is that they are the dark side of Getter Rays, evolution without purpose and running rampant. Their appearance with the advent of the Getter Furnace was not a coincidence, it was a catalyst. However, currently, only Professor Saotome knows of this. It's a probable theory that they may be the 'Spiral Nemesis', evolution gone mad, but they are more likely a byproduct of the worst aspects of life combined.
Combat
In battle, Invaders adapt to what the enemy is doing and usually fight them either on their own terms, if they have the means, or through possession and deception, if they do not. Generally they fight in some set form, but with the Invaders, you can never tell.
- Ground Invaders are usually quadrupedal and can raise to stand on two legs just as easily, with muscle distribution making them more bulky and resilient, with the ability to turn their limbs into blades and hooks, depending on the type of enemy they fight.
- Air Invaders are generally lighter and faster, with a large wingspan with talons in the end of them. Their primary mode of attack is to grab onto the enemy then sink all of their extremities in, then try to smother, crush or destroy their opponent.
- Space Invaders Usually come in all shapes and sizes, the most common one looking like a Starfish. These tend to be massive, since in space weight and mass is relative, and can use their hard bodies to ram into the enemy, or smother and crush them from all angles. It should be noted the smallest of these tends to dwarf a battleship.
- Metal Beasts are a new addition to the Invaders, ever since the Getter Bomb hit the planet. These Invaders possess discarded.. or still occupied.. robots used against them, replicate their weapons and fill out the parts, animating it themselves. This tends to give them new weapons, that came with the robot, as well as body armor and the ability to take much more punishment, letting the robotic parts take it instead of the less solid invader inside.
All Invaders are capable of energy projection of some kind.. usually some invisible pressure, like telekinesis, but the truly powerful ones can actually manipulate outside energies, and deflect or redirect them, even all the way back to their point of origin. As with all things involving these adaptable monsters, extreme caution is required, and firing at them without heeding this fact could have catastrophic results.