Does anyone know the origin of the phrase "Don't look a gift horse in the mouth"?

When you are buying a horse (or as a matter of fact everything) you want to know that it is in good condition, one of the methods is looking on the horse teeth to determine its health and age, since you didn't pay for a gift, why check it up?

Explanation 1:
Laokoon, a priest of Apollo, who, on seeing the huge Wooden Horse at Troy, warned his people not to accept the gift of the invaders. He even went as far as to stick his spear into the wooden belly of the contraption, and the clank of swords and spears was heard to issue from within. For this defacement of the sacred horse, the gods punished Laokoon by sending two enormous serpents, which issued from the sea as Laokoon and his two sons were sacrificing to Apollo. The snakes wrapped around the boys first, then their father, and dragged the three to their painful death. Seeing this as a judgement on them by the gods for their doubt of the gift of the Greeks, the Trojans accepted the Wooden Horse, and brought it into their city. From Laokoon's advice comes of course the famous sayings 'Don't look a gift horse in the mouth' and 'Beware of Greeks bearing gifts.
Explanation 2:
St. Jerome first recorded this saying in the fifth century and it was then written as '...never inspect the teeth of a gift horse.' It means that one shouldn't find fault with anything that has been given or offered as a gift.

An explination of the origins of the term 'don't look a gift horse in the mouth' from a book entitled 'A Hog On Ice And Other Curious Expressions' by Chareles Earle Funk, Litt.D. This expression or proverb is so old that it's origin cannot be determined. It has been traced to the writings of St.Jerome, one of the Latin Fathersof the fourth century, who then labled it a common proverb. The expression , or a variant proverb, occurs in French, Italian, Spanish, and other languages of Europe. The referance is, of course, to the bad manners displayed by one who recieves a gift and examines it for defects. Up to a certain age, the age of a horse can be determined by looking at it's {teeth;} though it may appear to be young and frisky, the number or condition of the teeth may show it almost fit for nothing but the glue-works.