Tahnok were a breed of Bohrok. They are red and control Fire.
The Tahnok, like all Bohrok, originally began their lives as Av-Matoran. Upon the completetion of their destiny in life, they would be transported to the Bohrok Nest, on The Island of Mata Nui.
Following MakutaTeridax's defeat at the hands of the Toa Mata, he resorted to using his Sonic Abilities to created a signal that activated the Bohrok Swarms. As was their destiny, the Bohrok were supposed to return Mata Nui to its original state by destroying any changes the Matoran had made to it. However, as Teridax had awakened them at his command, the swarms were awakened while the Matoran were still on the island.