Before the 2004 storyline
Matoran into Toa, Toa into Turaga, Turaga into Legend. This model of understanding the way of the Bionicle was taught to us in our childhood, when the grass was greener, Miramax was still alive and the trilogy was but a duology. Vakama uttered the now sacred words "You are not the first Toa" at the end of 2003, leading us into the 2004 – 2005 prequel storyline about Metru Nui and the Toa Metru, the original forms of the current Turaga. However, according to the words of Alastair Swinnerton, this twist was a later addition and was missing from the original story bible when it was first written in 1999. So, the entire system of Matoran (back then Tohunga) becoming Toa and the Toa becoming Turaga wasn't part of the canon at the time. Those three were actually different species. The idea of making the Turaga former Toa came to be during 2003, when the next year's story was being developed.
The role of Rahi in the GSR world then and now
During the release of the first generation of Bionicle (2001-2010), Rahi were extremely interesting and rich invariety on the one hand and a very debatable topic qua their role in the general canon on the other. At this present moment we are left with an unsolved issue – in the modern canon, the existence of Rahi isn't justified by any means. They are just inhabitants of a fictional world that Makuta created from Viruses and Protodermis to inhabit the Matoran Universe and for personal reasons, e.g. to serve as guards. How the Matoran Universe's wellbeing was improved by this high population of creatures was never explained. It seems sometimes that the creation of Rahi was an end in and of itself for the insane Makuta, but explaining it bysaying that the Great Beings work in mysterious ways is too simple – which is why this question remains so prevalent in the general fan community.
