Mr Khan said:
Metroid Prime (the creature itself) was the original Metroid, not a Metroid that became a Leviathan. The Chozo encountered this beast on Tallon IV, but in turn saw that it could be useful, and bred the race of Metroids off of that template for use in combatting the threat of the X Parasites that they encountered on SR388. I doubt that the other Metroids were bred straight off Dark Samus, since they were called Tallon Metroids, i imagine they were meant to be of the stock bred on Tallon IV |
I guess that makes more sense, but the fact remains that all the Metroids in that game, excepting Prime, all were from the one pirate frigate which evacuated with them off of Zebes.