The True origins of tunnelers are not known. it is  speculated that they may be a result of the Great War.  After the bombs fell, radiation heavily mutated them, adapting them for an underground life. These mutations resulted in a form of bio-luminescence. They evolved into quadrupedal predators that lived and bred beneath the sands of the scar. There they remained, completely unaware of the surface world for nearly two centuries.

Tunnelers appear to be reptilian-humanoids with dark, scaly skin, large bio-luminescent eyes, a mouth filled with sharp teeth, and chitinous spikes protruding from their shoulders and heads. Tunnelers live underground, and are averse to intense sensory stimulation. This can be taken advantage of through the use of flash bangs or firing a flare gun, causing them to flee from combat and head back underground. Their enlarged eyes appear to be an adaptation to the conditions of their habitat. The tunneler's bite is known to be as deadly as a bark scorpion sting.

Variants.

Tunnelers. 

These are the most common type of tunneler and will  typically attack in groups, swarming anything in their way.

Hulking Tunnelers.

Hulking tunnelers are the second largest variant of tunneler, second only to the tunneler queen. They are quite rare and are not as fast as regular tunnelers.

Venomous Tunneler.
The smallest of the tunnelers, they are also quite rare except inside a Tunneler Hive, when they are more frequent in the pack. Venomous tunnelers are faster than others and as their name implies, they possess a deadly venom which can quickly overpower an inexperienced adventurer. As they swarm with regular and hulking tunnelers.

Tunneler Queen.
The largest of the tunnelers, the queen combines deadly speed and Ferocious attacks, enough to kill even a deathclaw.