The phrase don’t look a gift horse in the mouth means don’t find fault with something that you have discovered or been given.

In A Concise Dictionary of Phrase and Fable (London, 1993), B. A. Phythian explains:

A young horse is a more desirable gift than an old one. A horse’s teeth reveal its age, just as old people without dental care suffer from receding gums and become long in the tooth. The sense of the expression, therefore, is that if you receive a horse as a gift, it is bad manners to look in its mouth to establish its value.

The original versions of the phrase refer to given horse. In A dialogue conteinyng the nomber in effect of all the prouerbes in the englishe tongue compacte in a matter concernyng two maner of mariages (London, 1546), the English playwright and epigrammatist John Heywood (circa 1496-circa 1578) wrote:

Where gyfts be gyuen freely, est west north or south,
No man ought to loke a geuen hors in the mouth.

In The Proverbs, Epigrams, and Miscellanies of John Heywood (London, 1906), John Farmer mentioned the following from Vulgaria Stambrigi (circa 1510):

A gyven hors may not be loked in the tethe.

As early as 387-88, the doctor of the Church St. Jerome (circa 347-420) had quoted the expression as being proverbial in the Prologue to Commentariorum in Epistolam ad Ephesios (Commentary on the Epistle to the Ephesians):

Non digne Græca in Latinum transfero : aut Græcos lege (si ejusdem linguæ habes scientiam) ; aut si tantum Latinus es, noli de gratuito munere judicare, et, ut vulgare proverbium est : Equi dentes inspicere donati.
     translation:
I don’t translate Greek into Latin properly? Then either read the Greeks (if you have a knowledge of their language) or if you are only a Latin, don’t criticise a gift gratuitously bestowed, and, as the common proverb goes, [do not] inspect the teeth of a given horse.

In Adagia (Adages – 1508), the Dutch humanist and scholar Desiderius Erasmus (circa 1469-1536) commented on equi dentes inspicere donati:

Hanc hominum incivilitatem quam incusat Hieronymus, nos nuper et experti et admirati sumus. Dedimus Novum Testamentum, innumeros locos vel emendavimus vel explicuimus, idque non aestimandis vigiliis. Incredibile dictu ut obstrepuerint, ut reclamarint theologi quidam ac monachorum vulgus, quibus is meus labor plurimum adiumenti adferebat. Quorum ut quisque maxime egebat hac mea industria, ita odiosissime reclamabat ingratum hominum genus, quibus si quispiam largiatur prandiolum aliquanto lautius unde distenti ac temulenti discedant, tantum non adorant hominem, et qui tantam utilitatem gratis donat, tot sudoribus constantem, etiam conviciis insectantur.
     translation by Margaret Mann Phillips – Cambridge University Press, 1964:
This same ungraciousness of men, which Jerome complains about, I have experienced myself, and marvelled at it. I gave them the New Testament; I emended and explained many passages, at the cost of incalculable toil. It was extraordinary how I was contradicted and shouted down by some theologians, and by the general crowd of monks, the very people to whom that labour of mine was of most use. That diligence of mine was conspicuously lacking to every one of them, but nevertheless they opposed me violently, the ungrateful lot—people who will almost worship anyone who hands them out something in the way of a luxurious meal, which they leave over-fed and tipsy. But the man who freely gives them so useful a possession, established with so much toil—on him they rain abuse.

Other Latin forms of the proverb existed. For example, the following, with reference to St. Jerome, is from Adagiorum chiliades tres, sive sententiæ proverbiales Græcæ, Latinæ & Belgicæ (Three thousands of adages, or proverbial Greek, Latin and Belgian phrases – Amsterdam, 1670), by Johannes Sartorius (1500-1570?) and Cornelis Schrevel (1615-1664):

Non oportet dentes inspicere equi donati.
One must not inspect the teeth of a given horse.

In Erasmus over Nederlandsche spreekwoorden en spreekwoordelijke uitdrukkingen van zijnen tijd (Erasmus on Dutch proverbs and proverbial expressions of his time – Utrecht, 1873), Dr. W. H. D. Suringar recorded other forms, among which:

Si tibi do mannos*, numeres ne dentibus annos.
If I give you small horses, don’t count the years by the teeth.

(* mannos, accusative plural of mannus, denoting a kind of small Gallic horse, a coach-horse used especially for pleasure-drives)

The proverb is found in many other languages; for example:
– French: à cheval donné on ne regarde pas les dents
– Spanish: a caballo regalado no le mires el diente, or, no se le miran los dientes
– Portuguese : a cavalo dado não se olha o dente
– Italian: a caval donato non si guarda in bocca
– German: einem geschenkten Gaul schaut man nicht ins Maul