Since IAMCOURIER nothing is scarier, removing myself would make....Tunnelers the scariest things.
The live underground, hunt in packs, are super aggressive, are dark green with glowing body parts and snake like slits for eyes, breed like rabbits and thanks to the Divide incident they could be in the Mojave in months. Not to mention they could take down Deathclaws in their pack and get around through tunnels making them hard to detect.
APPROVED.
Apart from the courier, who can nuclear bomb an entire nation of people, I'd say either Tunnelers or probably Cazadores. (I know x.x I probably spelled those wrong.)
Why?:
The courier is bad, really.. bad.. and they can release a nuclear bomb on a nation
(If they wanted to... )
Tunnelers like she said travel right under you and are insanely difficult to detect (Unless maybe the courier can sense movments in the earth) as well as wanna kill you in groups, and able to take down a full grown Death Claw as long as they're together.
Cazadores are simply a nucience really, hard as hell to hit, fast, and poisonous. Come in groups, and can fly.
Its more of neck to neck with Tunnelers being the snakes and the Cazadores being the hawks. It'd be very interesting to see who won seeing how you get a natural collision of land and sky which is bound to bring conflict. Nether the less, would be very interesting...
We all who have Lonesome Road would know Deathclaws wouldn't make it if the natural balance of sky and earth got tipsy with two different creatures that can and will kill a Deathclaw. Sorry Deathclaws, you're screwed.