Protocairn

The Protocairns were arguably one of the most bizarre creatures ever seen on Metru Nui. 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 fuse together, forming new land in place of the old they destroyed. In fact, the Great Temple of Metru Nui had to be rebuilt on Protocairn-created land after they destroyed the original temple. They were extremely difficult to stop, although Vahki began to have success in driving them back a few centuries before the Great Cataclysm.

Trivia
Protocairns were able to be built by combining the Toa Hordika Vakama, Matau, and Whenua sets.