The usual "look" = give a look of disappointment or disapproval; show ingratitude. "Punch" would change the meaning to "actively express disappointment or disapproval; complain and object, etc.

Pretty tough on the horse. it was not offering itself as a gift.

@Greybeard The usual "look" = give the gift a critical look in order to get some idea of how much it's worth. The horse might --- but then again might not --- be long in the tooth and thus not worth as much as you might have at first thought.

@Nemo Pretty tough on the puncher too. I'm no expert on horses, but I think they have hard teeth.

@RosieF In decades of speaking English, I can say I have never heard the idea of valuing the gift as a primary meaning. You are right that the horse's age can be judged by its teeth, but looking it in the mouth amounts to being disapproving of/ungrateful for being given a horse. For example if I give you a free, blue Ferrari and you complain because you wanted a red one, you have looked a gift horse in the mouth.

phrases.org.uk/meanings/…

I thought you were supposed to punch a shark.

The original proverb means "don't find fault with a gift", not "don't damage a gift".

My understanding of "Don't look a gift horse in the mouth," originates with the practice of inspecting a horse's teeth prior to buying it to assure it is healthy. A gifted horse is free, so checking it's teeth implies you are nit-picking the quality of a gift. I've never heard the "punch" version.

FWIW, we have the same phrase in German (with "look"): Einem geschenkten Gaul schaut man nicht ins Maul.

Perfect example of a malapropism! :-)

@GuntramBlohm and it would lend itself to the same pun, if we wanted to ("Einem geschenkten Gaul haut man nicht aufs Maul" has a certain rhythm). Luckily we don't want to.

@GuntramBlohm. And in Swedish: "Skåda inte given häst i mun[-nen]." But I've never heard anything like "punch".

Google cuts to a claimed 69 distinct hits, so I'd say this is non-standard.

@GuntramBlohm And the same in Russian: "Дареному коню в зубы не смотрят" (one does not look into gift horse's teeth).

@GuntramBlohm Same in Portuguese: "a cavalo dado não se olha os dentes". Never heard the "punch" variant either - looks like a malapropism

Someone tries to spice up a well known bit of wisdom, and completely mutilated it in the process. This could lead to all sort of confusion, but we'll burn that bridge when we double-cross it.

@DWKraus I've always assumed it had something to do with the original Trojan Horse.

There are a lot of... creative... false etymologies floating around for common expressions. People for some reason derive a lot of pleasure from being able to say "well, actually", and the more-recent versions are often colorful enough to gain some traction on their own. Here's one I find particularly annoying. Don't believe any etymologies that come without sources, and the more interesting the explanation is, the less likely it is to be true.