Skultera

Skultera (called Skultetra in Japanese) is an aggressive species of fish that appears in various Metroid games.

These creatures are native to Zebes, and are mostly encountered in the planet's underground oceans of Maridia. They can also be seen in the pools of water from Crateria, near the Wrecked Ship. If Samus Aran approaches them, they will home in on her and attack her.

An untold number of Skulteras were captured and brought to the Biologic Space Laboratories. During the events of Metroid Fusion however, they are infected and/or mimicked by X Parasites; individually, they suffer no modifications (other than their color) in performance. But if a Skultera-X were to be near a free floating X parasite, the latter would automatically combine with it and form an unnatural, larger Skultera. The small ones can be defeated by rapid fire, but the larger ones have a thick bone growth on their heads, and are only vulnerable to fully charged beams or missiles. This is likely due to the abilities of the parasites modifying the genes of their victims. Sometimes, when killed, their inner X will combine with a Zebesian to create a Mermen Pirate.

Curiously, the large Skulteras featured in Fusion returned in the later released prequel Zero Mission as standard, Zebesian Skulteras. Despite being larger than those seen in Super Metroid, these fish are actually quite weaker than all other variants encountered in games chronologically following Zero Mission's events.

Skulteras appear once again in Metroid: Other M, in their first 3D appearance. They were first confirmed in pages of an interview in the Japanese magazine Famitsu, which had a small screenshot of two Skultera on one page. They swim around various flooded areas of the Cryosphere and Biosphere and can latch onto Samus and gnaw on her.

Super Metroid manual
"These creatures are fish with bony frames and joints that rotate mechanically."

Super Metroid Nintendo Player's Guide
"These strong fish swim back-and-forth."

Aquatic Dangers
"Many of the Bottle Ship areas were intentionally flooded and my reconnaissance of them nearly proved to be fatal. Schools of Skulteras attacked me at every turn, and the liquid environments slowed my movements to a crawl. Fortunately, the Power Suit's air supply made breathing the one thing I didn't need to worry about."