"Rock icicle" is a subterranean species on SR388. The only occurrence of an official name of this creature exists in Japanese Metroid II: Return of Samus guides. It appears to crawl out of an orifice in the cavern ceilings, then drops down in an attempt to hit Samus Aran. These odd, fish-like creatures live in caverns that formerly contained SR388's acid. Their attack pattern is similar to that of a Skree in the manner of dropping down from above; however, it does not explode upon contact with the ground. It appears to live in some sort of family group, due to the seemingly infinite number of them that drop from a single spot. Strangely, these creatures stop crawling out of the orifice as soon as Samus kills a single one of them.
This is the only non-Metroid creature on SR388 that does not have any mentioning in the game's instruction manual, have any official artwork, or be mentioned in official English walkthroughs. It is not even mentioned in the Japanese version's instruction manual. This could point to the creature being either a very late addition to the game, or something that was originally meant to be removed, but was not. However, issue 31 of Nintendo Power refers to these as "Gravitts" on page 56. This is an error. ("Keep an eye open for Gravitts as you wind down this passage. They hang onto the ceiling and drop as you pass under them. You'll be able to avoid contact with them if you're quick.")