This is not an official term.
Many years before the Toa Metru's adventures, some specimens of ancient sea Rahi of tremendous size made their way to the ocean surrounding Metru Nui. They were captured and placed in the Archives for research purposes as part of a secret project.1, 2, 3 Turaga Dume authorized Mavrah's researches into the ancient sea Rahi.4 When it became obvious that keeping the beasts confined was a threat to the Archives and the rest of Metru Nui, Turaga Dume took the unusual step of ordering that they be driven from Metru Nui and prevented from returning. He instructed the Chief Archivist to put an end to Mavrah's researches, ordering the project shut down and the Rahi driven back out to sea by the Vahki. Before this could happen, Mavrah, unwilling to give up his work, allowed them to escape captivity and smuggled the creatures out of the Archives himself and through a tunnel in the Great Barrier. There he took up residence in nearby tunnels in a cavern while the Rahi lived in an underground river, where they lived in relative peace. The Rahi were not seen anywhere around Metru Nui since that time. Mavrah also allied with a small number of Kralhi, mechanical order enforcers which had been driven from the city long before.3, 5, 4, 2, 6, 1
The Toa Metru later traveled the river where Mavrah lived, taking their six Matoran spheres as they searched for a new land where the Matoran could live.3, 7, 8 They witnessed Takea sharks attacking prehistoric beasts along the underground river linking Metru Nui to Mata Nui.9 The arrival of the Toa Metru shattered the peace of Mavrah's hideout. Suspecting that the Toa Metru had come to harm the Rahi, and believing the Toa Metru to be agents of Dume sent to take him back, Mavrah ordered the Rahi, as well as the Kralhi, to attack them in an attempt to kill them.1, 3, 7, 8 Whenua was deeply disturbed by the fact that his old companion had become either badly misguided or insane.8 In the middle of a multi-way battle between the Toa, Rahi, Vahki, and Kralhi, Toa Onewa forced Mavrah to see the destruction his methods were causing. Guilt-ridden and dismayed, Mavrah rushed to the water's edge, calling for the fighting to cease. However, the battle had turned the river waters violent, and Mavrah was washed away by a sudden wave into the underground river. Onewa prevented Whenua from sacrificing his life in a vain attempt to save Mavrah. Mavrah was presumed dead. The fate of the Rahi was unknown.3, 7, 8, 6, 1
Notes
The Rahi studied by Mavrah and others were the first Rahi created.10
The Makuta didn't experiment on the first Rahi to create new species.11