A Metroid Cocoon is a breeding hive for Infant Tallon Metroids in Metroid Prime 2: Echoes. They are only seen in Biostorage Station, and hang on ceilings. According to the scan, they are used to transport the Metroids. They are very resilient to attack, which somewhat contradicts the statement in their scan (although the scan only says they have no armour or shielding; they may just have a very tough outer layer). It is unknown if the cocoon is a natural hive for the Metroids, or if it has been engineered to transport the species by the Pirates; the latter suggestion seems to be the most likely, as it is hinted in the scan below.