I was just thinking if Nel is an espada, could they be Nel's former fraccion? they seem to be hollows but hey could they be just in disguise just like Nel was?
"I am the bone of my sword.
Steel is my body, and fire is my blood.
I have created over a thousand blades.
Unknown to death.
Nor known to life.
Have withstood pain to create many weapons.
Yet, those hands will never hold anything.
So as I pray, "Unlimited Blade Works."
Seems to me that they were just average hollows (I think Pesshe is an arrancar) who were hanging out in the desert, found Nell, and had her join them. Plus, Dondo is just a hollow, not an arrancar, and Pesshe has no zanpaktou, which means he wasn't a Numero.