FusionDex

Rhytrum was incredibly well armoured and tremendously strong, to the point nothing could defeat it. It went extinct when its head armour evolved to be so dense that there was no more room left in its skull for a brain.

Thanks to its gargantuan jaws, which could shred thick metal plates as if they were paper, it was invincible in the ancient world it once inhabited. Once it starts running, it doesn't stop.