Petrasyls are floating, jellyfish-like creatures indigenous to an unknown location of the Tetra Galaxy. They are usually found in groups of two or more of their own kind and are not overly aggressive. Although at first glance or at a distance they bear a somewhat similar appearance to Metroids and an even closer resemblance to Mochtroids, they are not related to either species; in fact, upon closer inspection, they can be seen as floating, jellyfish-like creatures. They are harmless from a distance, though they can secrete a toxin that can burn through many materials.
Petrasyls are hyperthermophiles and are capable of living within lava flows, as seen in the Thermic Vast of Alinos. They are easy to kill and will perish from as little as two shots from power beam technology. In some places, such as the aforementioned Thermic Vast, Petrasyls live en masse, and will continue to emerge from the lava pools no matter how many a Hunter manages to slaughter.
- Metroid Prime: Hunters (First appearance)