While the Metroid Prime instruction manual calls the Space Pirates "interstellar nomads", they were later given a homeworld in Metroid Prime 3: Corruption.
The Space Pirate designs for Metroid Prime have three-fingered hands, as opposed to the lobster claw "crab" type found in Super Metroid, Metroid Fusion, Metroid Zero Mission, Metroid: Other M, and the official Metroid Manga. The Pirates instead attack with beam weapons and Scythes. However, concept art showing the "crab" type can be found in Metroid Prime's instruction manual. All Space Pirate designs in Metroid Prime also have flaming eyes. These new designs might be explained away as a "Phazon mutation"...
Samus first fights the Space Pirates on the Frigate Orpheon introductory stage, but the Pirates she encounters have been wounded in combat with the Parasites. The real battles occur once Samus first enters the Hydra and Aether labs via the room Research Entrance. Here, a special version of their theme, "Space Pirates (Laboratory Gate)" plays as a tense introduction to the main battle theme.