"Prototype"

A Dark Hunter once had an experimental weapon (implied to be a Spear of Fusion, like the one Vezon carries, and fired it at two Toa (of Fire and Earth) just as they were merging together.

The result fused them permanently and drove them mad, and the being went on a rampage and disappeared. Centuries later, the being was found injured in a swamp; it was nursed back to health by the Hunters and recruited, given the codename Prototype.

Prototype still has some control over the Elements of Fire and Earth, and his organic parts have decomposed to the point that he is mostly machine. He is rather slow. His life force is tied to his armor, but after so long his armor is almost impenetrable. Prototype is sometimes used as a practice target for rookie Hunters, if they survive a fight with him, then they can handle their missions.