Protocairns were arguably one of the most bizarre creatures ever seen on Metru Nui. Resembling raptor-like creatures, Protocairns had two Rhotuka spinners on their bodies which had the capability to temporarily disrupt signals between the mind and body of the target, which would in turn cause them to collapse.
As the legend has it, the Protocairns were once a group of Matoran from another land who willingly exposed themselves to Energized Protodermis to gain more power. They were mutated into amphibious creatures.
Every hundred years or so after then, Protocairns arrived at various islands and caused enormous destruction, disintegrating large portions of the island's coast. Shortly after, the Protocairns died and their bodies fused together, forming new land in place of the old they destroyed.
At one point, the Protocairns arrived on the shores of Ga-Metru and began terrorizing it. They succeeded in destroying much, including the Great Temple. Eventually, the creatures died and merged into a new landmass. The Great Temple was rebuilt on the Protocairn-created land.