A nag is an informal or colloquial term for a horse that does not belong to a recognized or documented breed, or more broadly for any horse considered to be of inferior quality, poor condition, or advanced age. The word entered English by the fifteenth century from Middle Dutch or Middle Low German roots denoting a small horse. In contemporary usage the term carries predominantly pejorative connotations, contrasting with formally documented animals whose breeding is recorded in a studbook. A nag may lack a pedigree altogether or may descend from mixed stock without registry documentation. The term is occasionally applied to any horse regardless of breeding when the animal appears unsound, aged, or unfit for demanding work. In casual speech, nag is sometimes used neutrally for any horse, without implying inferiority. Horses described historically as nags were often small working animals used for light harness and riding tasks. See also documented bloodlines for what distinguishes a registered animal from mixed or unknown stock, and the purebred standard as the formal contrast to a horse of undocumented breeding.
Further Reading: The term’s history from Middle Dutch origins through modern usage is traced on Wikipedia’s entry for nag (horse).