I've been having some trouble lately understanding what a Rahi is, and what the Rahi Control power is.
Here, I've tried to define both of these.
Rahi is Matoran for "not us". However, the term is not applied to Vortixx, Skakdi, etc. It almost always refers to animals- non-sapient beings. There is no issue when creatures like Muaka or Energy Hounds are referred to as Rahi. The confusion comes when creatures such as Tahtorak, Krahka, Artahka Bulls, Keetongu, and the Bahrag are called Rahi.
These creatures are all intelligent and more like beings than animals, as they are stated to have Matoran-level intelligence. As such, they should not be defined as Rahi, if Rahi are just animals. I have thought of a different trait which they share with the unintelligent Rahi which could differentiate all Rahi from non-Rahi.
Rahi are biomechanical organisms whose species do not form complex societies as civilizations are defined.
A one way of deterimining this for each species would be their participation in cities. Krahka and the Artahka Bulls live on their own, and neither take part in nor form civilized structures such as cities. Because of the Tahtorak's size, they must migrate to find food, and therefore could not have settled down or built cities (assuming they had the dexterity to do so anyways). Since Keetongu's species used the Tahtorak as mounts, they presumably lived a nomadic life as well. Finally, the Bahrag lived alone in the Bohrok nest, if you discount the impersonal Bohrok who lack minds of their own.
Thus, Rahi can be defined as species in the MU which do not form the same types of societies and structures as species like Matoran, who therefore refer to them by the term "not us".
The Rahi Control power, however, must be using the definition of Rahi as "animal". The power first appeared as a Kraata power in 2003, before the term Rahi had ever been applied to an intelligent being (I don't think the Bahrag were said to have been thought of by the Matoran as Rahi until later). At the point that this power was made, Rahi were always animals. In BA 10 (Time Trap), Keetongu and Makuta Teridax fought. Teridax won, but he had to actually fight, which implies that he could not simply use his Rahi Control power on Keetongu.
Later, in Into the Darkness, Karzahni attacked Teridax with his army of Manas. Teridax took control of half of the Manas to make them attack the other half, before threatening to have the surviving Manas attack Karzahni. This shows that a powerful user of the Rahi Control power can control many very powerful Rahi, but not a single intelligent Rahi.
In addition, the fact that Teridax only controlled half the Manas suggests that there is a limit to how many Rahi he could control at once.
The idea that more powerful (but unintelligent) Rahi are harder to control would make sense, when the battle between the Toa Mangai and the Kanohi Dragon is considered. It would be ridiculous to suggest that a single Toa with the right Kanohi could have stopped the Dragon- if this was the case, why would eleven Toa show up to fight it?
Thus, the term Rahi does not refer to the intelligence of the species like the term animal does. On the other hand, the Mask of Mind Control uses the "animal" definition of Rahi, and this power is limited by the intelligence, number, and power of the target(s).
Sorry if this was too long. Thoughts? Criticisms?