The Keras were large coral crab Rahi that dwelled in the ocean off of Ga-Metru, and in the reefs near Ga-Wahi. They were ferocious when protecting their home and were very hostile in general. Unfortunately, they were too hostile to train, save a few tamed by Ga-Matoran during the Bohrok War.
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 had an army that partially consisted of Keras crabs that were slightly mutated by The Pit's mutagen and became even fiercer.