The "Sly cats" (Polish: "Psotnych kotów") were a group of (mainly political) prisoners in Kovir, released by King Esterad Thyssen to serve as replacements mercenary branches (see also Free Company) fighting against Nilfgaardians. Of course Kovir officially "was, is and will remain neutral," and did not support the Northern Kingdoms.
They were not anything like an organised group; the name was just used by King Thyssen for all released prisoners, as a reference to a parable by which his wife, Queen Zuleyka, brought the idea to him.