"What is this thing?"
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Different entities in the Metroid series have been referred to as Metroid larvae. You may be looking for:
The baby, the last living Metroid, as referred to by Super Metroid and Metroid Prime and Metroid Fusion: Prima's Official Strategy Guide.
Infant Metroid, the second stage in the Metroid life cycle, referred to as "a larval stage" by Adam Malkovich in Metroid: Other M
Metroid larva, the third stage in the Metroid life cycle, as referred to by Metroid: Samus Returns Official Guide.