Halzyns are a creature native to SR388 that fly with vigorous wing flaps. The hard shell located on each side of its body keeps it safe from Samus' attacks. A Halzyn flies slowly from side to side in a wave-like motion, but can slightly increase the speed of its movement pattern when it detects Samus to heighten its chance of colliding with her. Halzyns can only be harmed if they are shot directly from above or below with beams and missiles, thus hitting the main body that their shells do not protect.
The X Parasites infected and mimicked captured Halzyn specimens in Metroid Fusion. They were ony found in the sector of the BSL Station designed to replicate SR388's environment, SRX. The X mutated the species: Halzyn-X are larger and also boast differently shaped, spiked-shells that cover more of their bodies' surface, as well as having different colors. The infected/copies also have enhanced mobility, allowing them to quickly dive downward when a target moves under them in an attempt to tackle it with their shells. After they crash down, however, the weight of their shells makes it difficult to fly back upward; this forces them to perform flapping motions that greatly expose their vulnerable main bodies. It would seem that the enhanced mobility was gained at the sacrifice of the armors' durability, as they are now weak to missiles.
"If [sic] flies unsteadyly [sic] in the air. The hard shell on both sides of its body keeps it safe from beams fired its way."