No one quite knows what it is. All they know is not to go into the cave, and everyone who did wouldn't come out again. For years it dwelled in the cave, devouring anyone or thing that came in to it. Until one day, when a hero in gunmetal arrived. It came out of the cave! For centuries, local villagers had believed that it had been a large rahi. They were wrong. It was bipedal. It was intelligent. And, most of all, it could speak! It told the matoran that it did what it had to. It didn't want to eat. Multiple times, it couldn't sleep with what it had done. It said that they can consider it a friend. They did not. They cast it out. It agreed, because it realized the tragedies it had caused the village. It decided to travel with the gunmetal warrior.
Here it is
If you pull the one of the horns on the back of its head down, it will open its mouth to chew something or speak
What the Head Normally looks like
Those fin things can become weapons!
With the Warrior in Gunmetal
As you can see, it's kinda a Master Chief and Arbiter type of deal! Rahi? Toa? Beast? Self-MOC? Who knows?! Well... You guys might know that last one....