Frostelus

Icebergs float in the far northern waters around Metru Nui, having appeared suddenly following the earthquake that struck the city. What no one realizes is that these are not natural forms—they are, in fact, homes and “armor” for the beings called Frostelus. Somewhere on the border line between Rahi and Toa-Turaga-Matoran, Frostelus may be former victims of the Visorak or some previously undiscovered species. How they came to be so near Metru Nui, and what their intentions may be, remains unknown.

So far, most of what is known about the Frostelus is that Norik and Iruini of the Toa Hagah ran into one on their travels (bringing question as to whether the species is hostile or not), presumably defeating it using a combo move in which Iruini's spear creates a cyclone with which Norik fills with lava from his own spear, creating a lava tornado.

We also know that the Frostelus are extremely protective of their territories, as in BIONICLE Legends 4: Legacy of Evil, a group of Frostelus launched an attack on a nearby Toa fortress which was guarding the Makoki Stone. The attack eventually lead to the deaths of all the Toa except for Lhikan.

Trivia

 * According to the BIONICLE: Dark Hunters guide, the Dark Hunters are hoping to gain an alliance with these Rahi. "Subterranean" is being used to negotiate with them, on the grounds that "he is one of the few beings uglier than they are" - The Shadowed One.