According to the film's lore, the Sumatran Rat Monkey is the result of small tree monkeys having been raped by giant plague rats that swam ashore from a sinking ship.
The monkey looks more like a rat than an ape, and its most notable features remain its wide, black eyes and its two long front teeth. It is nearly hairless and its skin colour is a brownish grey. Its nosferatu-like fingers are essentially claws and it has a suprisingly large amount of strength - easily ripping apart a chimpanzee in the zoo sequence.
It is brought to life in a jerky stop motion style, as a homage to veteran effects master Ray Harryhausen. In the sequence before the credits of the film, a hunter and his assistant evade the natives of the island and carry the monkey away for delivery at the Wellington zoo. The ape, however, breaks out of its cage and bites the hunter - leaving the assistant to hack off all of his body appendages because the simian carries a virus that turns its victims into flesh eating zombies.