The first Rahi were sea beasts of tremendous size.
These creatures were created by the Great Beings 100,000 years ago. For a long time these creatures disappeared, until they wound up near the coast of Onu-Metru at some point long before the first appearances of the Morbuzahk. They were herded into the Archives so they could be studied by three Matoran, Whenua; Onepu; and Mavrah, by order of Turaga Dume.
Unfortunately, things didn't go as planned, because a giant serpent attempted to escape, destroying much of the Archives in the progress. Because of this incident, Dume ordered that the Rahi be removed from the city as soon as order was restored in the Archives (much to Marvah's disappointment). That night Marvah returned and somehow managed to herd the Rahi out and lead them to a river system in the Great Barrier (where he ran into the Kralhi).
One thousand years ago, after Teridax damaged Metru Nui and imprisoned the Matoran, the Toa Metru began their journey to find a new home. This journey brought them through the Great Barrier where they encountered the Rahi. At one point Marvah ordered his "friends" to attack the Toa, which they did and when fifteen Vahki got involved things really got worse. Eventually, the battle got out of hand and the Rahi started to attack each other. In an attempt to stop the fighting, Marvah was swept out into the water, where he was presumed dead.
The status of these Rahi is unknown, but most are most likely dead or wounded.