The Monster

The monster is a red, tentacled, amorphous being. It is covered in eyes and gaping mouths and can re-shape and manipulate its bodily structure at will, as shown when it creates a protective shell around its body or created sharp spikes all over itself. The creature itself is incredibly swift and fast, especially for its size.

Appearance
At the start of the game, the monster appears to be approximately fifty centimeters tall with retractable tentacles covering its body. As it consumes more and more victims, it starts to grow larger, covered in gaping jaws and staring green/yellow/red eyes. When it eats, the jaw expands and unhinges to reveal a mouth coated and lined with razor sharp fangs, much like a snake, causing the prey in its grasp immeasurable pain. One of the creature's most notable features isn't just its long grotesque body, but also its long tendrils which it is able to whip out at lightning quick speed. Its tentacles are very long, able to extend over several meters. The monster has three different types of tentacles: One type sticking to surfaces, allowing the monster to move; one endowed with dexterity, used to grab and manipulate objects/preys; and one used as weapons, launching themselves at high speed and with great strength, easily destroying heavily armored military mechs. The monster is capable of shape-shifting, allowing it to take a human form at the very end of the game.

The creature itself seems to be possibly a form of tubifex worm, as is implied throughout the game. It might explain the ability the monster has to turn into worms underwater, but it doesn't explain the monster's appearance and abilities. It is unclear what the monster's true species is.

Trivia

 * The Carrion doesn't really have a name, but most refer to it as "The Thing", "The Creature", "It", or "Carrion", the name of the game.
 * Carrion is widely compared to John Carpenter's movie, "The Thing".
 * Carrion by definition is the decaying carcass of a dead animal.
 * Its eye color changes with its evolution stage.
 * First stage: Green
 * Second stage: Yellow
 * Third stage: Red
 * The creature's origins are not truly known and likely never will be, possibilities include:
 * An unidentified species of Tubifex worm, as stated by examining part of the monster in the first flashback.
 * An alien from another planet, this is supported by its level of intelligence (described below). Possibilities for why it's on Earth are:
 * Crash landing, what caused the crash landing is unknown.
 * Sent to Earth to research humans or something else on Earth.
 * Sent to kill off humanity, possible reasons include:
 * Preserving the monster's species as their home planet isn't safe to be on anymore.
 * To conquer the planet.
 * Regardless of the reason, it was taken to a lab, how it got discovered is unknown.
 * The creature seems to be intelligent as shown when it was capable of pulling levers and knowing where and how to get its abilities, also when it uses the vents and tight spaces to ambush its preys.
 * The creature is very durable as it harvests electricity to gain energy, take multiple bullets and survive catching on fire for long amounts of time.
 * Its shell makes it tougher, it's shown to be capable of protecting the monster from explosives that could instantly kill it.
 * The monster is shown to be very strong - breaking through doors, wooden walls, metal walls, tough protected glass - It can choose to toss a human around like a rag doll with ease.
 * According to the animated trailer, the creature seems to be able to control its host without the long tentacle that follows behind it when you control a person in the game, or that the monster is hiding within the person and the person has free control over their body and was just heading to the lab without knowing the monster was inside them.
 * According to behind the screams, the monster claims it is an artist and loves art, though it isn't known to be canon.
 * According to the trailers, whenever someone is possessed by the monster, their nose starts bleeding, although this probably isn't a direct symptom, and more like a side effect of being stuffed with a mass two times their size.
 * The monster seems to want to live the life of a human, as seen in the flashbacks and ending of the game.
 * This is unlikely, as the end of the comics depicts it entering Seattle through the sewers and exiting through a sewer grate in a parking lot, though the comic ends before it shows the monster's purpose, so far the only theories are:
 * It wants to live as a human for reasons unknown, possibilities include:
 * Escape the scientists and lab due to inhumane treatment, this is supported by the store page description on Steam saying "Grow and evolve as you tear down this prison and acquire more and more devastating abilities on the path to retribution.". This is unlikely since in the comic the monster isn't disguised as a human before exiting the sewer, but this could be an oversight.
 * Curiosity as to what the outside world is like.
 * Research about something outside the lab, or research the monster would rather conduct in a less dangerous environment.
 * Contact the rest of its species for one of the reasons listed above, how the monsters would accomplish this is unknown and this theory is only possible if the monster isn't "an unidentified species of Tubifex worm" and is an alien.
 * It wants to devour the human population and possibly the rest of life.