In myth, the Greeks put an end to their war with the Trojans by leaving a large, model horse as a gift, apparently, before withdrawing their soldiers. The Trojans joyfully pulled it into their city but - sadly for them - it contained Greek soldiers who opened the gates for their colleagues and brought about the destruction of Troy. Thus, one had to beware of them even when bearing 'gifts'.
And the words quoted by Moppettshow above are those of a Trojan seer who was trying to persuade his fellow Trojans not to accept the horse I mentioned in my earlier answer.
The words are taken from Virgil's Aeneid and spoken by - now here I know I will not be allowed to print the actual name, so here's what it is - 'Laoc' followed by 'oon'. Unfortunately, the last four letters are not permitted here even when used perfectly innocently!
The words mean, "I fear the Greeks even (when) bearing gifts." Sadly his fellow citizens ignored him!
I think traditionally the one about not looking a gift horse in the mouth suggests that if the horse is free, then you shouldn't be checking whether it's got all its teeth, rather than looking to see if any Trojans happen to be hiding in its throat
Stavros, at 24-7 KEBAB-U-LIKE, Broad Street, Birmingham. Last year he gave me botulism, when I complained he gave his stock answer: "Beware of Greeks bearing gifts!"