Metroids are the creatures of the Metroid Universe. They endangered all life in the galaxy due to their living off the "life energy" of their prey.
The Metroids were created by the Chozo who genetically engineered the species as "Ultimate Warriors". All Metroids are incapable of producing offspring with the exception of their Queen. Her sole purpose is to increase the population of her species and this implies hive activity. There are many different types of mutations and evolutions across the galaxy. The Metroids' sole purpose was to prey upon "X Parasites", their original apex predators.
The species eventually became naturally extinct and all subsequent Metroids have been created in laboratories.