Bigbug

"Bigbug01" and "bigbug02" are the internal file names for two creatures in Metroid: Samus Returns. These large creatures appear in the backgrounds of Area 2, Area 5 and Area 6 on SR388, but are never directly encountered. This may suggest that they are not hostile creatures. They are original to Samus Returns and did not exist in Metroid II: Return of Samus, as the backgrounds in the latter are all completely black.

Bigbug01 is located in the first room of Area 6 and appears to be a massive, slug-like creature with several larger spikes on its head, and smaller spikes down its backside. It also has a single yellow eye. It slides slowly through the background.

Bigbug02 is a large, three-eyed, creature in the background in Area 2. It has a long red neck and turquiose legs or wings standing in a hunched-over, bipedal position. It is seen in two rooms on the western side of Area 2. In the first room, a long shaft with a waterfall, it is seen drinking from the water though it will leave when it spots Samus the first time she goes through the area, but will return and drink from the water and will no longer be bothered by her presence indicating it has become tolerant of her presence. In the next room, it can be seen near steam vents, doing nothing. In a third room two of them can be seen lying in profile in the background again doing nothing. Another Bigbug02 appears in a fourth room in Area 5 where it can be seen entering the water in the background.

There are two other large creatures in the background that do not have any discovered filenames: A large, worm-like creature appears tunneling through the background of Area 3, occasionally visible due to holes in the rock wall. It does not seem to have any sensory organs.

Other large, quadrupedal creatures with crystals on their backs and greatly varying size appear walking around in the background of Area 4. The crystals on the back are also seen at many other places in the area, heavily impliying that they are actually specimens of this creature.

None of these creatures are encountered as enemies or bosses later in the game. None of Samus' weapons can hurt them, not even the Power Bomb. Despite their large, slow-moving bodies, they seem to have successfully avoided predation by Metroids.