CR 1/8 is fine.
It would be nice to see the PDF description of Robertia first, but I've come up with this flavor text in the meantime:
This little creature resembles a cross between a lizard and a naked mole rat, with a parrot-like hooked beak and two fangs in its upper jaw.
Robertia is a dicynodont, an herbivorous therapsid (“mammal-like reptile”). Dicynodonts are characterized by their parrot-like beaks, two tusks in the upper jaw, and barrel-shaped bodies with splayed legs. Robertia is the smallest dicynodont, close in size to a ground squirrel.
Robertia are social animals, living in large communal burrows. Only one female, the queen, is capable of producing offspring; all other females (as well as most of the males) are sterile. Robertia spend most of the day underground to avoid the hot desert sun, but during the cooler parts of the day, multiple individuals leave the burrow to gather food, which they bring back to the burrow to share with other members of the colony. This lifestyle allows the colony to maximize its foraging success.
Robertia is a minuscule creature; the average individual is 8 inches long and weighs no more than a few ounces.
Combat
A robertia encountered on the surface flees down the nearest hole if spooked, giving an alarm call to warn other colony members. If cornered, a robertia will bite.