“Where gifts be given freely—east, west, north or south—
No man ought to look a given horse in the mouth.”
(Attributed to John Heywood, 1497-1580}
We don’t have horses given to us very often, so the meaning of the phrase is probably quite obsolete. Its central idea was not to spend time inspecting a gift, but to be grateful for it.
However, if you can take the time to look a “gift horse in the mouth”, you may notice that some teeth are missing. Or the teeth may be there, but the gums are bleeding. So, in truth, perhaps you should look into the mouth of a gift horse.