Fenrakk was a monstrous mutant spider Rahi which served as a guardian of the Ignika during the Toa Inika's mission to retrieve the mask. It was mutated by the Ignika from a normal Fenrakk spawn to a larger size and fused to Vezon to serve as an additional guardian. It already possessed great strength and acidic saliva, then once fused shared Vezon's ability to absorb kinetic energy.
Vezon's monstrous creature, Fenrakk, pitted his incredible strength against the Toa Inika. When Vezon and Fenrakk were defeated, they rose again as Vezon and the Kardas dragon.
Under the terms of the Ignika's curse upon Vezon and Fenrakk, any defeat would result in Fenrakk mutating into a fiercer shape.
The Fenrakk spider was the partner/pet of Vezon. Like its master, it too could absorb the kinetic energy of blows directed against it and grow stronger. Fenrakk could not truly be beaten as long as Vezon wore the mask; if defeated it would simply transform into another kind of creature. Fenrakk possessed enormous strength and acidic saliva. It was defeated by the Toa Inika and transformed into the Kardas Dragon. Smaller versions of the Fenrakk spider were seen on Voya Nui, making it likely that this specimen was mutated by the Mask of Life.
When Vezon was fused to Fenrakk, his armor and organic components were part of Fenrakk's armor and organic components.
Vezon and Fenrakk had separate minds, they were not one entity.
Vezon and Fenrakk were fused together physically, they had individual consciousnesses.
Vezon and Fenrakk together were less powerful than Axonn, Brutaka, or Teridax.
Fenrakk shared Vezon's ability to absorb kinetic energy. Every blow struck at them or by them made them stronger.
Fenrakk could not understand the language Vezon spoke.
Fenrakk could not use Vezon's future sight.
Fenrakk spawn were not as strong as the enlarged Fenrakk.
Fenrakk was a male Rahi.
The Ignika didn't keep Vezon and Fenrakk alive under normal circumstances, but it would under other circumstances.