"Creature" was a term used to refer to an animal or other lifeform which came from a different time period than modern times. The term referred to either creatures from the distant past which were extinct in the present, or creatures from the future which had yet to evolve. Though these prehistoric and futuristic creatures were naturally only found in their respective home times, Anomalies could allow creatures to cross between eras and invade other times such as the present.
Other terms, used by non-British Anomaly projects, to refer to creatures included "erratics" (used by the Tunguskan Research Team) and "dinosaurs" (used by the Special Projects Group as an umbrella term for creatures from the past). (Extinction Event, Primeval: New World)