Why would any sane person want to look in a horse’s mouth? And what on earth would such behavior mean if a person were inclined to look in the mouth of a horse given as a gift anyway (so common right)? According to various sources, if a person is given a horse as a gift, it is impolite to look in the horse’s mouth to gage its age or other details relative to value. (An interesting corollary might be asking the price of something.) The essence of the saying urges acceptance of a gift for what it is and to be grateful.
Wow. This idiom must be old if it dates back to when horses were more common in everyday life, maybe the 1900’s or earlier. Would you believe 1546 or even circa AD 400, according to The Phrase Finder, Don't look a gift horse in the mouth.. Citations of historic references are given in the Phrase Finder write up as well as ongoing relevance. But in this day of cynicism, openness and examination of everything, how likely are gifts to be unchallenged or unquestioned? For example, political or business gifts would probably not cut muster.
Like other facets of life, there is the threat of obsolescence. Recent additional examples include hand writing versus key strokes, or internet versus shopping malls, and personal computers or tablets. Things seem to be moving pretty fast at times, especially with technology.
But it would seem a need for shopping malls will always exist on some level, as the preference for a personal computer and handwriting. So it is with being grateful and giving thanks, even expressed somewhat differently. It is timeless.