An unnamed pack of Gray Gorillas acted as the main antagonists of the novel, Congo, and it's film adaptation.
Being of a completely unknown species not yet discovered by mainstream science, they had an unnatural level of aggression towards humans which other great apes generally lack: to the point they would viciously murder any humans who intruded upon their home.
Their species live in and around the fictional city ruins of Zinj near the fictional volcano called, Mt. Mukenko, set deep in the Congo rainforest of the Great Rift Valley.
In the novel, it is made clear that they are not "true" gorillas by modern biological standards, but gorilla-chimpanzee, possibly gorilla-human, hybrids with physical features similar to that of modern humans and chimps. Their mass and height is closer to humans than gorillas, their skull is greatly malformed (the "ridge" that makes gorilla heads look "pointy" is nearly nonexistent) as well as their pigmentation is on the border of albinism: light gray fur and yellow eyes.
In the film, many exhibit some sort of skin condition similar to eczema, psoriasis, or icthyosis, resulting in large patches of dry, scaly skin rashes most common on the face-as well as crooked and broken teeth. In addition, they exhibit different behavior: they are highly aggressive, ruthless and partially nocturnal. Unlike normal gorillas, the Gray Gorillas are also extremely social, a troop consisting of over a hundred, compared to a normal troop of a dozen animals.
They are weapon users as well in the novel, fashioning crude stone paddles that they use to smash the heads of victims in a cymbal-banging motion. In the movie, they simply beat victims to death with their bare hands. Primatologist Dr. Peter Elliot intends to name them Gorilla elliotensis after himself.
The original human inhabitants of Zinj, as shown through a series of bas-relief frescoes, kept the apes captive, training and likely even breeding them to be hyper-violent and to attack humans. This was done to essentially make them guard animals of the city's valuable diamond mines; they would be relentless, uncompromising, and unable to be bribed by thieves or other intruders.
The legends of Zinj tell that the caravan route through the dense jungle was a closely-guarded secret back during the city's heyday, with foreign traders being blindfolded during the trek through the forest. With the roads into the city guarded by the warriors of Zinj by day, and the Gray Gorillas killing any living thing that entered the surrounding jungle by night, it ensured that no thieves or spies could escape with either diamonds or knowledge of the city's location.
In the film, many of the walls of Zinj have a series of hieroglyphs repeating the same phrase over and over: "We Are Watching". It's unknown if this message is intended to warn against thievery, akin to a "Beware of Dog" sign, or if it is a message to the Gray Gorillas themselves: "Master is Watching You".
It is unclear what exactly happened towards the end of the civilization which inhabited Zinj; whether the human masters lost control of their Gray Gorilla guards in an uprising and were all killed and/or driven away, or the humans simply abandoned the city when the mines ran dry. Eventually, the city was reclaimed by the jungle, and became the primary territory of the Gray Gorillas, who continued to teach their violent guard behaviors to their offspring, passing these behaviors down through subsequent generations. It is presumed that multiple expeditions sent to find Zinj were killed by the gorillas upon reaching the city, as none of these expeditions had ever returned before the 1970s. The city became a fable, a legend among Arab slave traders and natives living at the edge of the rainforest; a city of fabulous wealth guarded by evil demon spirits of the forest who smash the bones of intruders.
They are extremely territorial towards most other animals and will even prey on other species of gorillas, as evidenced by a crushed gorilla skull in one of their known killing grounds, and the characteristic head crushing seem in a photo of the corpse of Amy the Gorilla's mother. The edges of their territory are often marked by clearings where the ground is littered with the shattered and crushed bones of various animals, humans among them.
The Gray Gorillas seek to kill the expedition of scientists from Earth Resource Technology Services Inc. (Travicom in the film), led by Karen Ross, who are trying to find out what happened to a previous expedition to Zinj, who were killed by the pack while trying to discover a source of naturally boron-doped Type IIb diamonds, which can be used as semiconductors in computers. In the Novel, it is revealed that the Gray Gorillas have a complex vocal language which sounds like wheezing. After several more attacks, Elliot, with the help of Amy, finds a way to translate the language of the new gorillas (she refers to them as "bad gorillas") and piece together three messages ("go away", "don't come", "here bad"); they stop fighting the humans and become confused, leaving the camp. Most attacks come at night, and one group is observed passively foraging for food in the daytime. It can be assumed that their nocturnal aggression is a learned behavior taught by their former human masters, as potential diamond thieves would likely attempt to use the cover of darkness to escape Zinj with their stolen loot. The Gray Gorillas also do not attack the camp when the floodlights are on, waiting for them to be shut off before moving in to attack.
In the film, the dwindling Ross expedition stumble upon the mines themselves. Romanian philanthropist Herkermer Homolka, who co-funded the expedition, tries to take several of the large diamonds, triggering a killing frenzy in the Gray Gorillas. They kill Homolka, and seize Dr. Elliot, but his trained gorilla Amy rushes to his defense, signing at the "ugly gorillas" to "go away!", with her electronic translator reciting the message in a monotone computer voice. This action greatly confuses the Gray Gorillas, as they are unused to a normal gorilla standing up to them, protecting a human, or emitting strange spoken noises.
In both the film and the novel, the nearby volcano eventually starts to erupt and both the city, most of the diamonds, and the entire Gray Gorilla species was wiped out in the ensuing lava flow.