This locates the Rephaim at that time in Bashan, E of the Jordan.

More than 400 years later, just before Israel entered Canaan, “the land of the Rephaim” was still identified with Bashan. There the Israelites defeated Og the king of Bashan (De 3:3, 11, 13; Jos 12:4; 13:12),

Presumably it got its name from the tall people named the Rephaim who must have lived there at one time.