Nala was a king of Nishadas
Vrihadaswa said, "There was a king named Nala, the son of Virasena. And he was strong, and handsome, and well-versed in (the knowledge of) horses, and possessed of every desirable accomplishment. And he was at the head of all the kings, even like the lord of the celestials. And exalted over all, he resembled the sun in glory. And he was the king of the Nishadhas, intent on the welfare of the Brahmanas, versed in the Vedas, and possessed of heroism. ..."
Who exactly are the Nishadas in this context?