From Tar Valon Library
Author: Leora Oldessroth
The books never explicitly states who Arad is. After Mat shouts out in the Old Tongue when fleeing from Trollocs, Moiraine tells him that "The blood of Arad's line is still strong in the Two Rivers". This makes it very likely that Arad was a King of Manetheren.
(Reference: The Eye of the World, Chapter 18)