They are known as Vulptices or a Vulptex
Vulptices are native creatures of the mineral planet Crait. They live in the caves and tunnels under the surface of the salt flats which cover Crait. After the departure of the rebels from their base on Crait allowed the Vulptices to roam through the structures left behind by the rebels and the caves behind the base.
This free-roam allowed them to
Help the resistance escape the First Order by going through the caves and showing them the way out.
In an interview with EW, the head of the practical effects creature shop, Neal Scanlan, discusses how they got their crystalline nature from consuming the salt that covers the planet.
The idea is that these wonderful sort of feral creatures had lived on this planet and had consumed the planet’s surface, and as such had become crystalline... The designers took inspiration from “crystal glass chandeliers and the sort of luminosity and elements of refraction” they create.
Interview by EW with Neal Scanlan
According to the same article, the name came from Pablo Hildago galacticisin the latin name for fox, "Vulpes".