Although the Tolkien mythos gives the possibility of Mearas being descended from Nahar (or perhaps the steeds of Feanor and his brothers). Mearas may have been descended from Nahar but, Nahar was not a Mearas (does that make sense).think it was Nahar, the horse of Orome, who named it. Might be wrong though.
Excellent guess, but not the answer I am looking for.