Metroids are fictional parasitic organisms from the Metroid video game series created by Nintendo. They resemble jellyfish and can drain the life energy from other beings, making them a significant threat. Originally discovered on the planet SR388, they were intended to be studied for their potential to cure diseases. In the games, the protagonist, Samus Aran, often battles these creatures to prevent them from falling into the wrong hands. The Metroid life cycle includes several stages, from larva to adult, each with unique abilities. Their existence raises ethical questions about scientific experimentation and the consequences of tampering with nature.