It is speculated that the hammerheads are creatures produced by the scourge of the Ogdru Hem.
They are very large pseudo-hexapods with a bone-white exoskeleton. This armour could hold off most small arms fire, but explosives could break through. Direct shots in the mouth can also kill the creature.
The Hammerheads could survive in a variety of habitats, from the frozen tundra to scorching deserts.