Alien (Apollo 18)

These creatures are the only known form of life to be found on the inhospitable surface of Earth's sole major satellite, the Moon. In a resting state, they appear indistinguishable from simple lunar rocks; they also carry a powerful venom which induces psychosis and violent aggression in Humans.

Biology
When not moving, the carapaces of these crab-like creatures can close around their bodies to disguise them as rocks. When mobile or hunting, they utilize six spider-like appendages and two large pincers to catch and incapacitate potential prey. These creatures come in a variety of shapes and sizes, from rocks small enough to fit in ones hand to boulders large enough to be mistaken for large, stony outcroppings. This camouflage mechanism is curious, as it implies that the creatures either needed it to hunt or to hide from larger lunar predators. However, there is no other observable life on the moon to speak of, and seemingly non to be found in our solar system, at least as we understand life. This perhaps indicates that they did not originate on Earth's moon or that there are other creatures on Luna that were not revealed or rather chose not to reveal themselves to the crew of the Apollo 18.

These creatures carry a highly potent venom that they inject when biting, similar to earth spiders or ants. This potent toxin is almost one hundred percent fatal unless the effected limb is removed, though even this may do nothing to halt its progress, as it is unknown how quickly it can move throughout a terrestrial circulatory system. The toxin has many symptoms, including: blackening of the veins around the bite area (possibly necrosis), bloodshot eyes, severely antisocial behavior, and finally violent psychosis, ensuring that even if the venom itself does not kill the host, the host will kill themselves.

In the light these creatures remain dormant; this seems to indicate a partial reliance on solar radiation for sustanence, similar to photosynthesis. Bringing into consideration the lack of resources the moon would require to support life to support life, this is plausible. Staying dormant in the day could also help prevent DNA damage from intense ultraviolet light exposure from the sun, as the moon has no electromagnetic sphere to prevent these harmful rays from bombarding it's surface. Though the creatures are seen moving in the daylight, they seem to do it only when they are forced to do so. They will not move or twitch, even when being handled and relocated by unsuspecting lifeforms; a few were brought onto the lander as 'samples' and didn't shed their disguise until significantly later, after the sun went down. When the sun vanishes, they become active. The freezing temperatures of the moon at night likely inspire the arthropods to seek warmer spots and hunt down fresh prey.

The creatures also seem to communicate with each other through the use of radio waves, as using scents or calls is impossible due to the Moon's atmosphere being too thin to support such means of communication. This calling between the creatures is seen when their calls are picked up by the radio equipment on the lunar lander, which often disrupts the crew's connections with Earth and each other. This may also explain why the creatures are quite hostile to any of the equipment left by the Apollo 18 crew, as our own radio chatter could be painful or annoying to the creatures, inciting them to violence.

Smaller (perhaps juvenile) members of the species have been shown to burrow through flesh and become parasitic. It is interesting to note that this particular method of parasitic relationship is exceptionally crude and implies that the creatures do not have much experience attacking warm-blooded prey. The body of the Russian cosmonaut was severely desiccated, perhaps indicating that the creatures fed upon the man's vital fluids. This would explain the juvenile burrowing, as young require more food and warmth than matured individuals. It is likely that mothers of this species carry their newborn young within their carapace to keep warm, and so the creatures mistook the warm bodies of the astronauts for a female of their own species and were seeking shelter. Sometimes they will form colonies in impact craters where, in the dark, they can move around freely, even during daytime hours.