The Crites are from another unknown planet, and in the beginning of Critters, the last eight Crites are being transported to an intergalactic prison planet (presumably for preservation). There were ten Crites to be transferred, but two were killed as they were apparently eating everything in sight.
It appears they are an officially recognised species in the Intergalactic Council, as they have the reputation of making Intergalactic officials nervous because of their ferocity. They even have a policy on the extinction of any intergalactic lifeform, confirming that Crites are officially recognised.
The Crites are small aliens resembling porcupines or hedgehogs. They have large rotund bodies, covered in a haze of black fur. In the first film, the Crites' fur is styled so that their stomach is shown, but in subsequent films, it too is covered in fur. They have small limbs, with tiny claws on both hands and feet. A promninent feature is their large mouths, which take up the majority of their face and are filled with multiple rows of sharp teeth, which they use to attack and devour prey. They lack any visible nostril, but have a snout which is wide and flat, and they bear signature red eyes, which sometimes appear fluorescent from a distance. The Crites appear slightly larger in each sequel, and their fur appears more wild and spiky, as opposed to smooth, slicked back appearance in the first instalment. The hair is generally black in colour, but variations of brown and blue are not uncommon.
They've been described in various ways, such as being like rats, possums, Tasmanian devils, badgers and porcupines. Certain Crites have various unique features in the context of the film. For example, a Crite in Critters 3 rolls in some bleach, burning its face and bleaching part of its fur white. A Crite in Critters 2 has the hair on the top of its head shot off by one of the bounty hunters. The same thing happens to a Crite in Critters 4.
When first born however, Crites are understandably much smaller when fully grown, and their hair is significantly shorter. In order to grow to maturity, a Crite must feed at least once.
The Crites are notorious in the series for eating almost everything in sight voraciously, but in Critters 2, it is revealed that they are naturally carnivorous and prefer live meat.
In Critters, the Crites are seen eating electrical cables, stuffed animals, livestock, chickens and even a goldfish. The Crites in Critters 2 are seen eating a dog (as well as the dog food), wood, vegetables, the entire contents of a hamburger restaurant and even frozen burgers. In Critters 3, they feast on paint, mops, cakes, beans and even washing up liquid. In Critters 4, they consume some of the materials in an experimental lab.
When eating a meal, they usually do not leave a meal unless they are threatened or disturbed by humans, as confirmed by Charlie in Critters 4. The reason the Crites prematurely leave a meal in the same film can be explained by the Crites' eagerness to leave the space station to get to Earth (for a larger supply of food).
Crites behave mischievously, but have a vicious appetite that makes them deadly when hunting for food. Generally, Crites will attack in packs and one single Crite will initiate the attack, while others attack afterwards to bring prey down. In order to travel, Crites can walk, but will commonly tuck into a ball (like hedgehogs) and roll. They also have the ability to roll on the spot, gathering sufficient momentum to 'launch' from a position and reach quite long distances.
The largest Crite in the first film seems to take leadership of the pack, while the same happens in Critters 2 (although it is in fact an alien bounty hunter posing as a Crite). The bleached Crite seems to take leadership in Critters 3, while the bald Crite in Critters 4 takes dominance.
If Crites encounter more than one human, or they feel sufficiently threatened (such as a lone Crite encountering humans) they will generally shoot poisonous barbs that can knock out targets until the barb is removed. It is notable that in Critters 4, the Crites do not utilise their poisonous barbs.
In terms of intelligence, the Crites are more than capable to work together in order to get to their prey. In Critters 3, the Crites formulate a plan to gain access to a room through the ventilation ducts. Later on, they also deduce that they can use the elevator shaft in order to get to the humans near the roof. In Critters 2, when the Crites are trapped in a burning factory, they utilise a defensive mechanism in which the Crites combine into one single large Crite ball. This mechanism is used earlier in the film, although on a smaller scale, using four Crites. In Critters, it is apparent that they can perfectly work a spacecraft and even maliciously destroy a farmhouse using the ship's weapons. In Critters 4, the Crites reroute the crew's ship to go to Earth, and they also utilise a growth accelerator in order to rapidly breed more offspring.
In the first film, one particular Crite gets bigger (via eating), until it towers over the humans themselves. This feature was dropped in the sequels, but since the Crites of the first film were clearly older than the younger ones of the sequels, it may be a feature of advanced maturity.
Crites have to eat sufficiently before they lay eggs, but it is unclear how the Crites reproduce since it is never shown or discussed in the films. In Critters 4, Charlie tells Fran that it takes six months for Crite eggs to incubate, but this is contradicted in Critters 3, when the eggs laid at the beginning of the film hatch within the space of only a few hours. It is also unknown if the Crites have specific sexes, or whether they reproduce asexually. One theory is that crites are true hermaphrodites (being simultaneously fertile male and female at the same time). This would explain the rapid population growth displayed, since any mating would result in both individuals producing a clutch of eggs.
It can be assumed however that their fertility rate is high, given the large number of eggs present in the films. It can also be assumed that the Crites can self-replicate, since in Critters 4, Dr. McCormack is heard explaining that TerraCor is in search of 'a self-replicating predator species' to use in their growth accelerator, and it is suggested that the Crites are of that category, since TerraCor is intent on retrieving the last Crite eggs.