The armored Devonian fish with a bony head that could grow to about 6 meters long was Dunkleosteus. It was a giant arthrodire placoderm, an extinct group of heavily armored jawed fishes that dominated many Devonian seas.
Dunkleosteus did not have ordinary teeth. Instead, the front of its jaws carried sharp, self-sharpening bony plates that worked like shears. Its massive head armor was connected to the body by flexible joints, allowing it to open its jaws rapidly. Fossils of the head and jaw plates are common, but the rest of the body was made from less heavily mineralized tissue and is rarely preserved.
The genus lived roughly 382 to 358 million years ago, during the Late Devonian. Older reconstructions often portrayed the largest species, especially Dunkleosteus terrelli, as reaching around 6 meters or more. Recent estimates based on better body proportions have suggested smaller lengths for some specimens, so exact maximum size remains debated. This does not make the stored answer wrong: Dunkleosteus is the canonical answer to the description of a giant armored Devonian fish.