I was talking with my friend and I said that I needed to hear a fact straight from the person who said it, and then she said something like, “yeah, you have to hear it from the horse’s mouth.” I inquired what she meant by this, where she had heard it from, etc. This is what she told me.
Informant: “My mom says, ‘I heard it straight from the horse’s mouth’ and that means that you heard it from the person who said it, so it’s authentic.”
Collector: “Do you know why it’s specifically a horse?”
Informant: “I don’t know, but she did grow up around a lot of horses. She grew up on a cattle ranch. And they all rode horses around.”
Collector: “So do you think this is specific to farmer culture or rancher people, rather than city folk?”
Informant: “I think so because you tend to… your language is dependent on your surroundings. You use analogies based on where you live, or on the things that you know”
The informant didn’t know much more about the origins of the proverb, but after some basic online search, I found that thefreedictionary.com offers the following explanation: “this expression alludes to examining a horse’s teeth todetermine its age and hence its worth. [1920s]” As my informant mentioned, this expression probably originated from a culture that was accustomed to being around horses, so its relevance in the future might be questionable.