Keras

The Keras are large coral crab Rahi that formerly dwelled in the ocean off of Ga-Metru, and now dwell in reefs near Ga-Wahi. They are ferocious when protecting their home and are very hostile in general. Unfortunately, they are too hostile to train, save a few tamed by Ga-Matoran during the Bohrok War, so the Matoran cannot put them to as much use as Ussal crabs.

During the Bohrok War, Gahlok drove Keras out of the ocean and onto beaches, so Onu-Matoran Ussalry members volunteered to help the Ga-Matoran tame a few Keras to fight against the Bohrok.

These special tamed Keras were fitted with plates on their backs to make them easier to ride. They could carry their riders across sea and land. As long as they were provided with shelter from the sun and fish to eat, they were content to defend Ga-Koro.

Keras in The Pit
Barraki Carapar currently has an army that partially consists of Keras crabs that are slightly mutated by The Pit's mutagen and have become even fiercer.