IXTL

Planet of Origin: Unknown but presumably destroyed for a long time.
Ixtls are a very ancient sentient species, thought to be extinct, although some specimen could still be floating in the emptiness of space in a state of suspended animation. They are red colored, with four legs and four arms ending in eight fingers. Their cylindrical body is virtually immortal and they have in full control of its  molecular structure, enabling them to survive unprotected in space for eaons in suspended animation and allowing them to withstand very high energetic discharges. To reproduce, Ixtl need what they call a Guul, i.e.living hosts in which it deposits their eggs. The eggs need only a few hours to hatch and then feed their living host.
Source: A.E. Van Vogh, The voyage of the Space Beagle

ASGARDIANS

Planet of Origin: Unknown but their current place of residence is Asgard, a place situated in a wormhole network (of which Earth is part) englobing 9 worlds. Asgardians have developed the Bifrost technology enabling them to teleport themselves instantaneously to any of the 9 worlds using the subspace network of the wormhole.

Asgardians are a highly advanced humanoid species. Earth'sNorsemen and Germanic tribes used to worship the Asgardians nearly a millennium ago. Even though certain Gods are still interested in humanity such as Thor, the Asgardians do not have any more active worshippers or seek to have any. Asgardian flesh and bone is about three times denser than similar human tissue, contributing to the Asgardians superhuman strength and weight. An average male god can lift up to 30 tons while an average female can lift up to 25 tons, but there are those that exceed these specifications. They  are also gifted with extremely long life, but are not immortal. They are also immune to all terrestrial diseases and are highly resistant to conventional forms of injury. The metabolism of the Asgardians gives them superhuman endurance in all physical activities. Their close genetic and morphologic resemblance with humans leads to believe that those two species, along with the other ones of the the 9 realms could be of a same origin.

Asgardians residing on theworld of Vanaheim are refered to as Vanir while the ones residing in Asgard itself are refered to as Aesir.

The other species of the 9 realms are:

• The Dwarves: Dwarves are small and stocky with an average height of 4 feet. Dwarves live in Nidavellir and maintain peaceful relations with the Gods. They tend to be either craftsmen or farmers and have created weapons for use by the Gods such as Mjolnir and the spear, Gungnir.
  
  
• The Elves: The two types of elves that inhabit the land are the Light Elves and the Dark Elves. The Light Elves live in Alfheim while the Dark Elves live underground in Svartalfheim. Both species are similar in the respect that their body types can differ greatly in size anywhere from 4 feet to 8 feet, and have proportionally long limbs. Light Elves have lighter skin tones while Dark Elves are darker.
  
  
• The Demons lives in the realm of Muspelheim. Demons are similar in size to the Gods, but are engulfed in fire. They tend to be nomads but will travel into other dimensions for the purpose of war. All Demons are the offspring of the fire demon who is said to be older than asgaridn culture, Surtur.
  
  
• The Giants: the Giants live on Jotunheim. They are also humanoid, but are more Neanderthal-like in body structure. They are the traditional enemies of the asgardians with whom they fought a millenia-long war that ended in 965 after they tried to settle on Earth. They were repelled to their homeworld, which was devastated and left without means to travel using the Bifrost network. Most stand up to 20 feet tall, but some can reach 30 feet. Giants usually lead a hunter/gatherer type of life, but will war with the asgardians on occasion because they are jealous of their superior abilities. The first of the Giants was the Ice Giant, Ymir, who existed before any of the Asgardians. Many different races of Giants arose in Jotunheim, including the Ice, Mountain, Rime, Storm, and Frost Giants. The Frost Giants are the direct descendants of Ymir. The Giant Laufey was another monarch who lost the war  against the Asgardians; In tribute to Laufey's courage and to try to seal a peace treaty with tthe giants, the asgardian king Odin adopted the giant's infant son Loki. Two other giants of note were Fafnir II and Fasolt (both deceased), who built Odin's palace in Valhalla in exchange for a Goddess, and battled over a magic ring and treasure that was given for the Goddess being released. Giants have also been known to reproduce half-breeds that look like asgardians.
  
  
• The Trolls: Trolls are massive creatures covered in hair with an orange skin tone. They average in at 7 feet, and they live in settlements scattered throughout the land. The trolls are incredibly strong, surpassing the strength of the asgardians, and puts them on similar ground with the Giants.Thor. Trolls usually work as miners and metal workers.

Sources: Thor movie, Marvel Universe, More on Marvel.com: http://marvel.com/universe/Asgardians#ixzz1S2qPajjt

SKITTERS

Skitters, are an aggressive space faring species of six-legged reptilian clawed insectoid aliens with two prehensile arms. They are nocturnal, speak in a kind of strange language with a manbidle equipped mouth and possess light-green eyes with vertical black pupils. They appear to have armor that covers their body except for their head and legs. They invaded the Earth in the early 21st century and destroyed all major cities. At first they were not engaged because humanity thought they might be peaceful. They "harness" children using a centipede-like parasite to control the children's minds so they can communicate with humans. They use bipedal Mechs and airships. 

Source: Falling Skies series 

BATTLE LOS ANGELES ALIENS / BLAA

Planet of Origin: Unknown. Given the decrepit appearance of their ships and equipment, it's possible that the aliens that come to Earth in 2011 may not be part of the dominant force in their homeland and may be looking for a new home planet, possibly due to their old home being uninhabitable because of warfare or some other cataclysm. This would explain their War of the Worlds-esque landing method, as well as their relatively low level of technological advancement for a spacefaring civilization. It is theorized  that they are invading for Earth's liquid water.

Description: The Battle: Los Angeles Aliens (often refered to as Blaa) are an aggressive advanced spacefaring caste-based bipedal carbone-based organic species practicing heavy cybertransplant on themselves.They are divided in castes, easily recognisable by their morphological specificities and the nature of their implants. 

Four warrior castes are known to exist.The warriors are highly developed bipeds with four fingers hands among which are opposable thumbs. They sport redundant arteries and other blood vessels. Their heads are actually entirely sensory organs. Their brains are located in a cavity behind their hearts, directly attached to the spine. They have large numbers of stem cells and platelets, enabling them to regenerate whole limbs that may be lost in combat, while their hardened exoskeleton allows them to take heavy damage before being too injured to continue fighting. All warriors have substantial amounts of machinery implanted into their bodies, including communication, cooling, and weapons systems. Their perception is also very acute, supplementing their precognitive and retrocognitive abilities. 

Infantry Caste: The Infantry Caste can be identified by weapons grafted onto each arm where grasping hands would be. Their average height is about 8'1". While they are taller than humans, they are proportionately thinner. All of their vital organs are concentrated behind layers of muscle and arteries in the right of their chests. A cooling system is also integrated into their body, with a heat-dispersing fan located in front of their organs. Unlike many of the other castes, they do not have as much machinery located on their back.

Officer Caste: The Officer Caste is the easiest caste to differentiate from the rest, identifiable by their slender build and four mechanical legs replacing their biological ones, which enable them to move at speeds of up to 40kph/24.8mph with relative ease. They are also the tallest caste with an average height of 9'4". The Officers are specialized in commanding groups of infantry, augmented by an unusually large, well-developed brain and very keen eyesight which has been supplemented with additional electronics. The Officer caste have the most substantial amount of cybernetic enhancements. As their legs have been wholly replaced, they are able . Their right arm has an implanted weapons system that trades firepower for accuracy, while their left grasping hand has been replaced by a mechanical claw. 

Operations Caste: The Operations Caste share many similarities with the General Infantry Caste, with a few notable exceptions. The Operations average about 6'11". The special forces appear to be a more intelligent Infantry soldier, expected to undertake specialized goals beyond that of simple shock troops. They also appear to be a middling caste, lacking the firepower of the Infantry and the intelligence of the Officer. Unlike the officer and infantry castes, the Operations have retained their left grasping hand and are able to make use of hand signals. Their weapons systems sport greater accuracy than those of the general infantry, but a less-powerful grenade launcher.

Intelligence Caste: The Intelligence Caste is likely specialized in their namesake, gathering information and laying down lines of communication. Averaging about 5'10", the Intelligence caste are the shortest and stockiest of all known warrior castes. Unlike all other warrior castes, the Intelligence Caste lacks any integrated weapons systems. However, they retain the large amount of cooling and communications machinery grafted to their backs. The lack of weaponry is made up for by their ability to utilize both grasping hands, which they use to operate machinery and vehicles, such as driving ground artillery platforms and operating walking guns. 

Water Extractot

The full extent of their space-faring capabilities is not known. Most of their light weaponry uses projectiles, but they also use heavy energy weaponry. Although the known vehicles make use of different forms of propulsion, such as walkers and hovercrafts, none are wheeled. All are oriented toward greater mobility, at the cost of armor and therfore, the protection of the users. The have intensive use of cybernetic implants and replacement of actual body parts and do some genetic engineering to increase efficiency of castes,An extreme case of such engineering is the Water Extractor, a specialized alien, used as biological pump that is used to move massive amounts of water quickly, growing as large as 100 feet in height. The Extractor's organs have been greatly reduced and compacted, leaving only organs that are vital to life functions. Extractors are highly intelligent and use an advanced form of communication to coordinate their actions for maximum efficiency. The extractors are deployed along with the initial invasion force, landing off-shore and using modified tentacles to quickly pull water from the surrounding environment. 

The aliens  launched a surprise invasion on Earth, most likely motivated by a need for a new home or colony on the 12th of August, 2011. Originally mistaken by the Humans to be clusters of meteors, their  spacecraft landed off the coastal shorelines of twelve of the world's major population centers. Their ground forces subsequently attack many of Earth's major cities, including Paris, London, Moscow, New York, Hong Kong, Rio de Janeiro, and Tokyo. Their aircrafts eventually arrive and appear to easily take control of the air above the cities  from the human Air Force. Besides their ground and large air forces, they deployed a sort of command center controlling unmanned flying drones. In Los Angeles, within a few days, the drones  pushed most of the U.S. military force out of the city, killing thousands during the engagements but a squad of Marines  able to destroy the command center, disabling all of the drones and depriving them of their air assets. These Marines later retreat back to a command post, and join up with thousands of reinforcements ready to try and take back Los Angeles from the invaders. 

Souce: Battle Los Angeles, Battle Los Angeles wiki