Alien (Apollo 18)

These crab-like creatures are a deadly species native to Earth's moon. They can disguise themselves as rocks, and carry a deadly infection that causes infected humans to become extremely violent and psychotic.

Behavior and Anatomy
The rock creatures can sprout crab-like legs and pincers, and are otherwise indistinguishable from normal moon rocks. They vary in size from as big as boulders to small enough to fit in one's hand.

The creatures also carry a highly powerful and dangerous infection which they can infect human hosts with by attempting to burrow into their flesh. The infection will slowly spread throughout the host's body; black veins will appear on parts of the body that have been infected, and the host's eyes will turn extremely bloodshot. The host will slowly become more and more reserved and aggressive, and eventually enter a state of extreme psychosis. It is unknown whether or not the infection is fatal, as there are only two known victims of it; one died when his space helmet was broken, the other was killed by the rock creatures.

During the day, the creatures prefer to remain disguised as ordinary rocks, even when disturbed and/or in the presence of potential prey, but become active at night. When the creatures become active, they will pursue and attempt to kill anything (alive or inanimate) that disturbs them.

The smaller creatures, when they encounter and are disturbed by humans, will attempt to burrow into the human's flesh; thus, even if the host survives the creature's attack, they will fall prey to the infection that the creatures carry. It was also shown in one of the alternate endings for the Apollo 18 films that the bigger creatures will attempt to kill human victims by impaling them with their pincers.